Thursday, September 4, 2014

Flash Photography Basics

Flash Photography Basics, an Introduction


The reason to use flash is of course that flash is very bright (and very fast) for easy camera exposure. In comparison, the brightest light bulbs are dim for photography, far from sunlight bright. Without flash, even very well lighted rooms will suffer from long slow shutter speeds, or high ISO, or both. Light bulbs can be fine for still life photography, when a one second shutter is no problem, but which is unacceptable for pictures of people, who tend to move. Flash also allows us to create the lighting, to be like we want it, to place flash wherever we want them, and to be made soft as we might desire, etc.

Flash photography is many things. There is on-camera flash and off-camera flash, manual flash and automatic TTL flash, and direct flash and bounce flash. There is fill flash in bright sun, multiple flash units, studio and portrait and table top flash in umbrellas, high speed flash, and more. Lighting is a big and fun subject, but before anyone can get much into "lighting", there are a few more fundamental basics we need to know, about "light". In all of these cases, there are basic differences between flash and existing continuous ambient light. Flash is not difficult, it is just different than either sunlight or regular continuous room light, and we need to understand flash too.

In short summary, the major points, the really big deal about flash, is:
  • The intensity of any near light source (flash or continuous) falls off fast with distance (inverse square law). Therefore, flash can achieve a correct exposure AT ONLY ONE distance (flash to subject). TTL automation can determine that exposure, but we also need to know that relative to that subject distance, any distant background is necessarily underexposed, any close foreground is necessarily overexposed. Bounce flash can help to minimize this, but distance is a huge issue for flash, with huge implications concerning our use. But in drastic contrast, sunlight is quite unique, very special because the Sun is so distant that its intensity appears not to vary with subject distance (here on Earth). Do NOT assume flash acts the same - it doesn't. Flash is in the same room with us.
  • Flash pictures involve two exposures, flash and ambient, with two different concepts of rules (Part 4). The camera meters these separately, and we can control them individually.
  • Flash is typically very fast, shorter duration than the shutter duration (the shutter merely needs to be open when the flash occurs). Speedlights in particular can be very fast, easily stopping extreme motion. But therefore,flash exposure is not affected by shutter speed (Part 2). But of course, continuously available ambient light (continuous light) is still affected by shutter speed, like we always understood. Since flash exposure does notcare about shutter speed, but shutter speed does affect any continuous ambient light, then specifically, we can use shutter speed as a tool to adjust the ratio between flash and continuous light in our photos (Part 4).
  • Flash is convenient to modify the light itself, as we desire, for example, large close lights (umbrellas for example) are very soft light with vague diffused shadows, instead of the harsh dark shadows from a small light source (Part 3). Photography of course has other important factors (composition, lighting, etc), but flash is simply about adding light, and flash exposure is simply about adjusting the flash power level to deliver the right amount of light to the specific distance of your subject.
  • Flash imposes a few limits we work around, like flash power capability and flash range is limited. Also our maximum shutter speed has a limit (maximum shutter sync speed, which varies with shutter, but usually in the ballpark of around 1/200 second maximum shutter sync speed). And waiting for recycle time between pictures can be a factor. But flash also allows us control over more things about the light in our photos - direction, intensity, soft light from umbrellas, etc.
I just have to say that this really is all easy stuff, and I suggest that it could help to try just a little before letting your eyes glaze over. :) Photographers simply need to know. Many of us just do just "want the picture" without any bother. But it is about like just wanting to drive - we did have to learn to operate the car, and we did have to learn the rules of the road, and maybe some about ice and snow too. So we learned a few easy things, and now we've got it for life. So yes, there are a few details to know about flash too, and we may actually have to think a little now and then, but none of it is difficult. The rewards are great.

Flash is just a light that we can aim. In one way, it is just another light source, but we can aim flash where we want it (lighting), and we can turn its power up or down (exposure), to deliver the lighting and exposure we want. It is not rocket science. Our picture shows everything that happens. For Exposure, we simply adjust the flash intensity to give the result we want at the subject. In manual flash modes, we simply adjust flash power level to do this. In TTL flash modes, TTL automation gets it close, and then we simply adjust Flash Compensation to adjust this level for our preference. Either way, manual or TTL, if it is too bright, then turn it down, etc.

"Lighting" is a skill to be developed (which is mostly about learning to actually "see" the results, to notice, to be aware), but the flash is just a light, and we can see exactly what it does (if we just bother to look at the result, carefully). Anything you cannot see should not matter in a picture, however, we do learn to observe more with experience. Much of the problem is that beginners probably have not yet learned to see much, and you should be aware that the ability to "see" is the main thing to be learned about lighting. There is a page about that too . Flash is just additional illumination, however we can aim and modify the light to be like we want it to be, and we can learn to control the shadows it makes. This concern is not so much speaking of the shadows behind the subject, because generally we often try to eliminate those. This is speaking of the shadows and soft gradient tones ON the subject's face, intentionally created by off-camera lighting (including bounce flash).

Flash can be necessary, and it can be a big help. The simplest tips for universally better hot shoe speedlight snapshots (the way we ought to be using our speedlights, but sadly, many don't know yet) are:
  • Pictures indoors need flash of course, and specifically, Bounce Flash offers much better lighting. Aimimg the flash head up at the ceiling is simple, not always possible, but better lighting is almost automatic when it is possible. Direct flash is flat uninteresting light, but bounce is from an off-camera angle, causing soft graduated tonal shading that shows shapes, and is greatly improved. Turn off Auto ISO, camera mode A or M, ISO 400 and f/5, and flash TTL mode mostly takes care of bounce automatically today (under white ceilings, 8 or 10 feet, or maybe 12 feet high). Pull out the white bounce card (card should be "as small as is usable", definitely NOT "large as possible". Too much card obliterates the bounce, and just becomes flat direct flash again). Don't stand too close (6 or 8 feet, then zoom in if desired). It is all good, but bounce does need more flash power.
  • Pictures of people outdoors in bright sun need fill flash, to lighten the dark harsh shadows. Balanced flash mode is designed to do this, and even the little popup flash will help, if the distance is not too great. More detail belowand here. Or of course, finding some shade is always good (softer light), but a little fill flash is still needed. In bright sun, camera P mode with flash TTL BL mode can take care of it automatically today.
  • The control of automatic TTL exposure is done with Flash Compensation. Simply adjust Flash Compensation as seen needed. If TTL automation gives too much flash, turn it down a little with -EV Flash Compensation, or vice versa.
You should try these things, the results will be self-evident. There is a detail or two though, and if your experience level is comfortable working with aperture and shutter speed, and if you want to be able to manage your flash pictures, then the material here is what you need to know. If still a beginner, and not yet comfortable, you really do need to know, and there is a very useful book here. These first basics are the dividing line where we become knowledgeable, no longer clueless. We won't be able to go very far without fundamentals.

These flash basics here are about "light itself", and are obviously important even if you always use automatic TTL metered flash.

Some people avoid any math. Inverse square law and guide number concepts do have a little simple arithmetic, easy stuff. It will not do you any harm by simply reading it. We don't actually do any math, but we need to understand the concepts, about what to expect. And relax, only this page and the guide number page have any of the simple arithmetic (multiplication and division). So the following pages are better, but the first concern might be "how bright is my flash?"

1. Inverse Square Law

Light intensity falls off rapidly with distance from its source. This is called theInverse Square Law, which says the intensity varies with the square of the flash-to-subject distance, this way:

  • Light at 2x the distance is 1/4 as bright, and light at 1/2 the distance is 4x brighter (2 stops)
  • Light at 3x the distance is 1/9 as bright, and light at 1/3 the distance is 9x brighter (8x is 3 stops)
  • Light at 4x the distance is 1/16 as bright, and light at 1/4 the distance is 16x brighter (4 stops), etc.


Inverse Square Law is just a fancy name for a rather simple concept. Think of a flashlight - as the beam travels farther away from the source, the beam spreads out to illuminate a larger area, but becoming more dim with distance. All light spreads this way, your flashlight, your table lamp, and your photo flash, all weaken in this way too. We might imagine that if the light were twice as far away, it would be half as bright, but the correct answer is only 1/4 as bright. The drawing explains why it falls off so fast. At twice the distance, the coverage dimensions do double, both width and height, so the lighted area becomes 4x larger, and the light distribution is 1/4 as bright. The Inverse Square Law is only saying that the light spreads to cover a larger area as it travels.

This drawing is from the Wikipedia topic. It shows that when an angle spreads in space (a beam of light), and travels twice as far (2r vs. r in drawing), the Width and Height of its beam spreads to be twice as large (Similar Triangles). That 2W x 2H expands to 4x times the first Area, which still contains the same light, but which is therefore diluted to be only 1/4 as strong at 2x distance (same answer if we compute a circular beam). The illumination varies with the square of the distance (varies inversely - more distance is a weaker light).

We can suppose the red lines are the paths of a few photons of light traveling from the source. Photons don't become weaker with distance - the angle of the beam just spreads out. The greater area dilutes the light intensity - the same photons in a greater area, so less light per unit of area, simply because the light is the same energy distributed over a larger increasing area. Nine photons at 1x, distance dilutes density to about two per area at 2x, which is 1/4, and one per area at 3x, which is 1/9. Impressive little drawing!

It really is that simple, that is all there is to it. The inverse square law is only about the spread of any angle, and is not about any property of light at all. The effect is the same on light, gravity, sound, and radio waves, because it is only about the angle and distance. Angles just spread out with distance, and any light just fills that larger area, and thus is weaker intensity (metered at any one spot). You already know this, a flashlight beam becomes dim with distance because it spreads out with distance, becoming more dim. We might imagine twice as far is half as bright, but the big deal is that in fact, it is only 1/4 as bright there, explained above. So the point is, light falls off fastwith distance, more so up close, but the amount varies inversely with the square of the distance. Your photo flash is a light, and it does this too. It is good to realize this.

We may not care why, but it definitely matters to photographers that this does happen. All you really need to realize is that subject distance from the flash is a huge factor, like shown in the yellow chart above. However, a confusion is that sunshine seems to be a major exception - direct bright sun appears to be constant brightness no matter where we stand, independent of distance to the subject.

Sunshine is quite special (due only to our own local situation). Sunshine does of course work exactly according to the inverse square law too, there can be no exceptions. Yet sunshine seems very different, and actually appears NOT to work that way. However, it is the distance to the light source that matters, NOT the distance to the camera. Sunshine seems to have a constant brightness anywhere we look, which is only because we are 93 million miles from the Sun, and another few miles to yonder mountain we see here on Earth is a totally insignificant difference. Even the 240,000 miles to the Moon is insignificant (1/4 of 1%), so the astronauts could use the same Sunny 16 rule there that we use here. On Mars however (half again farther from the Sun than the Earth), they will have open up about one more full stop (inverse square law). But since we cannot vary our distance from the sun source here on Earth, sunlight does in fact appear uniquely constant to us - only because the sun is always same distance from any subject here on Earth. This can give photographers false notions about how other light ought to work, but it is the Sun's distance that is the exception. The flash is in the same room with us, only a few feet from the subject, so we WILL see the Inverse Square Law in action. It is the overwhelmingly huge and major factor for our flash use. We might work with it without knowing exact details, but we absolutely must recognize it exists.

The exposure does not depend on where the camera is, or how far the camera is from the subject (unless the flash is on the camera). What matters is how far the flash is from the subject.

This is yet another confusion, another classic paradox, about how flash distance greatly affects exposure, but camera distance does not. It is enough to know it is true. Frankly, this topic may better be omitted for beginners, and instruction sources always do skip it. Yet, we may be puzzled about why camera distance does not affect exposure? Harder to explain, and it iscovered here, if you must, but that explanation seems an advanced topic, not essential. Don't let it distract the pursuit of flash basics. What we need to know is that flash intensity falls off fast with distance, according to the inverse square law.

Exposure variation due to flash-to-subject distance

Since intensity at the subject varies with distance from the light source, an implication is that any flash exposure can only be "correct" at one distance from the light source. Stop and think about that a second, it is an essential to know, a biggie. This Inverse Square Law (light falloff with the square of the distance) is true of all light, any light, a table lamp or a campfire at night, etc, but using flash for photos is commonly where this becomes more important to us to know. We cannot "fix" this Inverse Square Law situation, nor can we ignore it - we can only learn to work with it.

The Inverse Square law explains why the room is seen to be darker behind nearby people in a snapshot using direct flash. The distant background obviously has to be darker, it is farther from the flash (just how life is). There are ways to help this situation. Flash pictures are double exposures, of flash and ambient. Using a slow shutter speed will aid bringing the low room light level up, at risk of motion blurring the image. Or using high ISO will aid bringing the low room light level up to match the flash. Both methods are at risk of the incandescent light causing a strong orange cast (high ISO flash pictures often will require a CTO filter on the flash, so Incandescent white balance can be used with flash). Often far best, simply using ceiling bounce flash greatly helps to minimize this distance difference, since most parts of the (small) room are more equal-distant from the ceiling. Or in studio situations, another light is commonly used to illuminate the background area.


It is quite important to expect and plan on this distance variation for flash. Again, it does not matter to lighting where the camera is, but pay attention to distance between flash and subject. Arrange your subject, or look for a lighting angle for the flash, so that all parts of your subject are near the same distance from the flash.

I'm just saying, if your picture and subject has a camera angle something like this sketch, then a frontal flash will be different illumination levels at the three subject distances. If using only one flash, then consider a flash arrangement like shown here, to illuminate the subjects evenly. And of course, bounce from the ceiling comes to mind too (or maybe bounce from the left wall, aimed at a spot about where this flash is shown now). The three subjects will be more evenly illuminated when equal distant from the flash, regardless of where the camera is. Or if multiple distances are necessarily involved, consider more flash units to illuminate each area - for example, another light on the background for portraits. Otherwise, that is why a white background half again farther than the subject will be underexposed about one stop, and will appear gray, not white. White backgrounds pretty much require their own light, to show as white.

If Manual flash, we just adjust the flash power level to produce what we want, for the best photo exposure result. For one flash, this can easily be trial and error, judged in the camera's rear LCD, or aided by the histogram. Or we can use a handheld flash meter to meter and set the power level of multiple lights, each set to known ratio values relative to each other. Metering is much faster for multiple lights, instead of guessing at trial and error multiple times. Each light can simply be set precisely, so we actually know what each light is doing, and then we can easily repeat the same setup exactly next time.

For TTL flash, exposure is automatically metered, but when we discover we need a bit more or less flash than the automation provides, then Flash Compensation is the way we control TTL flash. Which is very large part of any success, and is easily the best single tip about using flash - if you don't get the result you want, don't just bemoan your fate, that never helps. Do something - Fix it, then and there. Simply adjust it until you see what you want. Flash Compensation is the tool to adjust what TTL automation does. However, flash does have some different basic properties (discussed here), which are good to know to use it.

This calculator computes the stops of light falloff between any two distances from direct flash. Distances can be any units (feet or meters), but the two must be the same units (it is a ratio). Inverse Square Law says double the distance is two stops less light.

For reference, we know that one stop of exposure is a 2x brightness difference, and two stops is 4x.

A rough guide to estimate the light falloff is this: Suppose the subject is at 8 feet from the direct flash, and the flash picture is setup to be correctly exposed there. Then we can be certain that background objects at 11 feet will be underexposed 1 stop, and objects at 16 feet will be underexposed 2 stops. Foreground objects at 5.6 feet will be one stop overexposed, and objects at 4 feet will be 2 stops overexposed. You recognize those example distancenumbers (4, 5.6, 8, 11, 16) as being f/stop numbers, and coincidentally (simply due to both definitions using squares), this aperture scale we have memorized provides a good quickie guide to estimate this falloff.

F/stops are not distance of course, but if the flash distance (feet or meters) approximates any f/stop number, then half or double brightness corresponds at distances of "one stop intervals". This is merely coincidentally true (we know f/stops are not distances), but nevertheless it works, a reasonable guide. More technically and precisely, the square root of 2, or 1.414 times the distance is one stop down, and 2 times the distance is 1/4 power, or 2 stops down - which just coincidentally agrees with the f/stop numbers - each f/stop number is 1.414 times the next one (incidentally, if you pursue this, what we call f/11 is mathematically f/11.3. We simply say f/11 for round off convenience - see more about these values).

So this inverse square relationship also implies that if the flash is close, its illumination falls off fast at close distances behind the subject. If the flash is farther, it is already weaker of course, and since twice a far distance is farther than twice a close distance, decline requires more feet, but same percentage. Either way, close or far, if at twice that distance, the flash will be 1/4 as bright, which is two stops down. That is a big deal, we notice that.

So there is an additional rough guide about "depth" of the light field. Direct flash light falloff has a small range around the subject which might be usable. TTL automation might follow a moving subject, but manual flash will need to keep the subject in this small range. Or the subject might have this much shape extent itself. We saw from the rough guide that if the light is at 4 feet, then it is a stop wrong at 2.8 or 5.6 feet. In that case (4 feet), one foot difference varies the light nearly a stop. But if that light is at 11 feet, then the one stop difference is at 8 or 16 feet. One stop wrong is still a large problem, but the 1/3 stop values either side of f/11 are f/10.1 and f/12.7 - so we know in this case (f/11), the range is around plus or minus one foot around f/11, for a 1/3 stop difference. If your subject has to move (dancing or kids), then this gives a good clue how much range you might tolerate around the median distance. But better, TTL flash is very good for such moving targets, since it keeps remetering the current situation.

Effective Flash power


Quick notes about the relative scale of things related to flash power. Some random facts, cute facts even, but which ought to become obvious to your understanding.

Stopping the aperture down one stop (like from f/4 to f/5.6) requires double flash power. Two stops is 4x power, and three stops is 8x power. Speedlights often don't have enough power to do low ISO bounce at much more than about f/4. ISO 400 f/4 is generally a safe try.

Changing manual flash power level to half or double the previous power level is a one stop difference. The marked manual power levels of Full, 1/2, 1/4, 1/8, 1/16, 1/32, 1/64 are full stop steps. So increasing the flash power by one stop simply means to double the previous power level (like from 1/8 power to 1/4 power is double power). If you reduce your flash power to half of any previous level (like from 1/16 to 1/32 power), you can open one stop of aperture to compensate (from f/5.6 to f/4).

Increasing ISO to double value (like ISO 200 to ISO 400) requires only half of the flash power (one stop). Doing that ISO double twice (ISO 200 to ISO 800) requires only 1/4 the power (2 stops). One implication when buying lights is that this means that a 160 watt second flash at ISO 200 is exactly the same lighting situation as 320 watt seconds at ISO 100 - both at same power level setting will use the same f/stop for same exposure at same distance.

Increasing flash-to-subject distance by 1.414 (square root of 2) times more distance requires double flash power (one stop). Two times the distance needs 4x power (inverse square law), which is two stops.

The Guide Number (next page) of multiple equal flashes used in combination as one, is (GN of one) times sqrt(number of flashes).
Two equal flashes are one stop more power than one flash. Four flashes is two stops. Eight flashes is three stops, etc.

About adjusting settings for TTL flash:

Note that when using automatic TTL flash, the automatic flash metering will simply respond to any changes with a different level of flash power, trying to keep the same correct exposure.

For TTL flash, changing aperture, or ISO, or distance, or adding a diffuser, or bounce, or an umbrella, just changes power level, NOT exposure.
TTL compensates the exposure with power, tries to keep exposure right. That's what TTL does.

So - Flash Compensation is instead the tool we use to control what automatic TTL does, to adjust the automatic flash exposure.

However, for Manual flash mode, changing aperture or distance or ISO or modifier changes flash exposure of course, unless we manually compensate the power level ourself.

White Balance with Flash:

Flash White Balance is very nearly the same as Daylight White Balance, however the color temperature of the flash varies a little with flash power level - Color is NOT a constant. Ionization spectrum in flash tubes depends on the level of electrical current through the flash tube (power). Speedlights adjust power by truncating the duration, to be shorter, but necessarily more reddish at high power level (full low level trailing discharge tail is retained), and more bluish at low power level (red tail chopped off). Here is obvious evidence of that, which you can repeat yourself. Most studio lights are the opposite, most adjusting power with voltage level, becoming reddish at low power levels. Flash tube color simply varies with adjusted power level (just how it is). However, the Paul Buff Einstein lights are an exception, which combine these two methods in a calibrated way, so that one trait offsets the other, for a more constant color at all power levels (but you still have to match that one color to your cameras White Balance). It is extremely convenient in studio sessions to include a White Balance Card in the first test pictures, to easily and trivially correct the color of the pictures later (also suggesting Raw is very helpful).

Exposure when adding multiple lights:

Combining two equal flashes directed at the same subject area from same distance is double power level, and twice more powerful is one stop. Four equal flashes doubles again, to two stops. Eight flashes is three stops. Each double is one stop.

Combining two flashes of unequal power will still add, and two will be brighter than the brightest alone, but two (even if equal) are never more than 2x brighter than one (all else the same, distance, etc). Your Main/Fill light situation for portraits will meter 1/3 or 2/3 stop more (depending on lighting ratio) than just the Main light alone (so if you set camera aperture to what just the main light meters, you will overexpose a little). So meter both main and fill together, to set the camera aperture. FWIW, the math is that if we have main at f/8, and fill at f/5.6, they add to be sqrt(8² + 5.6²) = f/9.76 (but just meter them together).

See Part 4 for more about fill flash in bright sun.

Inverse square law with umbrellas and softboxes:

Note that for any Inverse Square Law distance computation (where half the distance is expected to give two stops more intensity), we cannot measure distance from the fabric. The distance should be measured from the real light source, which is from the flash tube to subject. For a softbox or shoot-through umbrella, the distance is from the flash tube through the fabric to the subject (distance of subject to light stand pole is close). For a reflected umbrella, this is the distance from the flash tube to the umbrella fabric, and then back to the subject (includes two trips along the umbrella shaft). The fabric is not the source of light, it is merely a bump in the path that looks that way. Out in front of the fabric, this bump is just a constant power step, as if we just turned the light down.

And yes, I do know this is not what is taught in school (they feel a compulsion to measure from the fabric panel, perhaps convenient - if it worked - but which is not the source of a softbox). So there are plenty of doubters (who apparently never actually tried this). Yes, I do understand that when up close, all the points out towards the edge of the large panel also contribute added light inwards to the center line. Yes, measuring from the fabric certainly is a problem up fairly close. It is also true the center path is attenuated by the fabric, so there seems to be a compensation. Yes, there are ifs and buts and exceptions. But this is not a luminous panel - the light from behind is attenuated by the fabric. There are no great new scientific principles revealed here, just that if you insist on using inverse square law from a wide source, you'll do much better measuring from the actual flash tube than from the fabric. Anyway, my notion is that inverse square law obviously does hold pretty close if simply measured from the (actual) flash tube source. Try it.

We can easily verify the truth of it. For example, an Alienbees B400 flash at 1/2 power in a 40x32 inch AB softbox (double baffled, internal nylon panel), metered at ISO 100 with a Sekonic L308S meter. Flash tube is 17 inches behind front fabric. Metered using a makeshift plumb bob string held in metering hand, over a long measuring tape on the floor (with its zero end directly under the flash tube).

Softbox Evidence: ISL at doubled distance should be 2 stops down. The closest reading (2 feet from flash tube) was seven inches in front of fabric.

Feet, Inches
from Tube          Meter
                          f/ + tenths         Stops
32     384           f1 + 0.7            2.1
16     192           f2 + 0.8            1.9
8         96           f4 + 0.7            2
4         48           f8 + 0.7            2
2         24           f16 + 0.7          0

The one 16 foot reading was 0.1 stop high, which was repeatable. The chart sure would look beautiful if that one were 0.1 less. But which is only 0.1 stop, and 2, 4, 8, and 32 feet were right on, within 0.1 stop, and 16 feet is extremely close, no more than 0.1 stop. There was an obstacle at the side, in front of that point, probably it created a little added reflection? The stated accuracy of this Sekonic meter is ± 0.1 stop, and I was careful, but I'd suppose my procedure might cause another bit of error now and then. Switching to less precise third-stop metering might help hide or smooth tiny variations.

It sure does appear that metering from the flash tube obviously does follow inverse square law (from the actual light source). I've checked this several times, it always works for me, certainly close enough. There is no noticeable discrepancy. Try it yourself.

Of course, it is much easier to simply meter the lights, than to worry with this

Wednesday, August 20, 2014

The Digital Lens Calculator

How to know the right lens for every photograph

©1995-2013 Fred Parker

THE CHALLENGE

One trait that separates professional level photographers from amateurs is the ability to pre-visualize a photograph. Sure, you have every focal length lens from 6mm to 1200mm. But you can't carry all of those lenses with you all of the time. As you move through your life, most of the time you will not have a camera around your neck. But, if you have the soul of a professional, you will see photographic opportunities everywhere. Many will be missed entirely because of the fleeting nature of life experiences. However, several times a week you will be standing in a location, repeating the recurring chant of the photographer: "Wow! What a great shot! I wish I had my camera!" The fact is, if the "shot" is compelling enough, you will return. You'll think about the best time of day/season for lighting, make a note of your position and come back at the perfect time.
But, wait! Haven't you forgot something? Which of your drawer full of lenses will you bring with you? If you're like most photographers, you'll bring a bag full of lenses, but not the right one. The challenge is to come back at precisely the right time with only those lenses that you need. How can you do it? Read on!

BE WELL ARMED WITH DIGITS

All you need to estimate the proper focal length lens is your arm and digits (fingers, that is - remember, this is aDIGITAL lens calculator). If you're a REAL stickler for accuracy, you can bring along a very small (one meter) tape measure calibrated in millimeters. You see, Nature must have had photographers in mind when she designed Homo sapiens. Not only that, she prefers the 35mm format for film. And the metric system.
I should explain. As it turns out, the long dimension of the 35mm format at various focal lengths from 50mm to 1200mm is (pretty exactly) represented by the anatomy of your hand stretched out to arms length.
For example, if you spread out your hand with your arm stretched out in front of you, the distance from the tip of your thumb to the tip of your little finger will equal the long dimension of a 100mm lens. A 200mm lens is represented by the length of your index (pointing) finger at arm's length, etc. The table below gives you hand measurements for most lenses from normal through extreme telephoto.
But there's more! At the distance of an outstretched arm, 10mm equals one degree of field of view. Since you already know the angular field of view for all of your lenses, figuring which lens to use is a snap! Just bring along a meter tape measure. What's that!? You don't know the angle of view for all of your lenses? Well….let me give you a shortcut. A 50mm lens has a 40 degree angle of view. That's all you need to know. Double the focal length to 100mm and you cut the field of view in half (20 degrees). Double it again to 200mm and halve the angular distance again to 10 degrees. And so on. You get the point.  If you have very long or very short arms, please be aware that the finger measurements will be more accurate than the tape measure. That's because finger and arm length tend to scale proportionately, while a tape measure remains fixed. And, alas, 5% of the population will have long arms and small hands (or short arms with large hands). If you are in one of these groups, you'll have to make adjustments from the chart. This chart will be very accurate for 95% of photographers. 
Of course, all of this works only with the 35mm film format. And a yard stick won't work (it must be metric). The table below gives data for the most popular lenses in the 35mm format. If you have a lens with a focal length that falls between the ones in the chart, fake it. Please note that this is an anatomically correct calculator. A brief lesson in anatomy will help with the chart. If you look at one of your fingers (you choose) you'll find that it is made up of three segments (except the thumb, which has two). The term for each segment is "phalanx". Collectively, they are "phalanges". The distal phalanx is the one with the nail on it. The proximal phalanx is the one closest to your palm.


Lens
FocalLength

Frame Width ø

Frame Height ø

Frame Diagonal ø

Frame Width Arm's Length (mm)

Anthropological Measurement At Arm's Length (Approx)

17

93

70

104

930

20

84

62

94

840

24

74

53

84

740

28

65

46

75

650

35

54

38

63

540

50

40

27

46

400
Two Hands Spread

70

29

19

34

290

85

24

16

28

240

100

20

14

24

200
One Spread Hand

180

11.2

7.4

13.4

112

200

10

7

12

100
Index Finger, All Three Phalanges

300

6.5

4.35

8.1

65
Middle Finger, Proximal Phalanx

400

5.1

3.3

6.1

51
Middle Finger, Middle Phalanx

500

4

2.45

5

40
Thumb, Distal Phalanx

600

3.3

2.2

4.1

33
Middle Finger, Distal Phalanx

800

2.5

1.6

3

25
Little Finger, Distal Phalanx

1000

2

1.22

2.5

20
Width of Thumb Distal Phalanx

1200

1.45

1.1

2.05

14
Length of Little Finger Nail Quick to Fingertip

Estimating Lens Angle of View

THE ULTIMATE EXPOSURE COMPUTER

A GUIDE FOR NATURE PHOTOGRAPHERS
©1995-2013 Fred Parker

bullet

Introduction
bulletWhy Your Light Meter Lies To You
bulletExposure Value, Film Speed, Shutter Speed and f/stops -- What Are They?
bulletHow To Use The Ultimate Exposure Computer / Throw Away Your Light Meter!
bulletA Typical Day In The Life Of A Nature Photographer
bulletPlanning Ahead
bulletUsing The Ultimate Exposure Computer Wisely
bulletThe Final Step
bulletWhere on Earth Are You?
bulletExposure Value Chart
bulletExposure Factor Relationship Chart
bulletUseful Photographic Exposure Guidelines
bulletHandling Tricky Lighting Situations
bulletEV, Footcandles and LUX (For The Truly Dedicated)

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INTRODUCTION

Imagine an exposure computer so advanced that it uses your eyes as a sensor. The processing unit is as powerful as your brain. The computer is accurate over a light range from reflected starlight through the light produced in a hydrogen fusion reaction. This computer weighs nothing and operates without batteries. It comes with instructions to allow you to implant the capabilities of the computer directly into your own memory so you can accurately judge a correct exposure by simply looking at the type of light that the subject is in. You are using this computer right now!
Everything I've said above is true. However, I'm sure you've noticed that the only thing before your eyes is a World Wide Web page. An entire photographic industry has been built by convincing consumers that the subject of photographic exposure is so arcane, difficult and impossible to understand, that it is best left up to technology. This, of course, allows companies to sell billions of dollars worth of equipment to figure it all out for you.
The fact is that the concept of photographic exposure is extremely simple. It can be mastered by anyone who can multiply or divide by two. Everything you need to know is contained on these few Web pages.
Knowledge of photographic exposure is essential to controlling the creative side of the photographic process. This knowledge increases the chance that the photograph that comes back from the lab is the one you envisioned when you pushed the shutter release. Your camera or light meter can not make creative decisions for you. If you want to control the creative side of photography, you need to understand the interrelationship of Exposure Value, film speed, aperture and shutter speed. You must have this understanding even though you own the most sophisticated equipment available. I have watched photographers (amateur and professional) struggle with this subject for a number of years, confused by the misinformation that abounds in the photographic press. I decided to publish this document to clarify, simplify and demystify the issue.
But first, a word about copyright. This document is copyrighted. In order to keep the cost down, it has been published in a form that would be easy to copy. Copyright law strictly forbids copying this document. You may print a copy or two of the Ultimate Exposure Computer to keep in your camera bag for personal use. In fact, I would encourage it. All I ask is that you send me an e-mail (fred at fredparker dot com) to let me know that you've downloaded this document. If you get a chance, drop me another e-mail after you've worked with it and let me know if you have any suggestions. However, please do NOT make copies for your friends, students or any other person. Originals are inexpensive and easy to obtain. If you have a related site (commercial or otherwise) please hyperlink to this site for access to this document. If you download all or part of this document and post it directly to your site, it will be assumed that you have decided to purchase redistribution rights, which START at $3500. Thanks!

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WHY YOUR LIGHT METER LIES TO YOU

Light meters can be less accurate than you might imagine. Their response to various colors of light may not match that of your film. Their response in low light and high light conditions may not match response at the mid-range of light. But the worst characteristic, for practical purposes, is that all reflected light meters make one basic (erroneous) assumption. They are calibrated to render an exposure that will make the subject look like a middle tone in the resulting photograph.
How do you compensate for the fact that your meter is lying? You have to lie to your meter. If you are photographing a light toned subject, such as snow or sand in bright light, you must convince your meter that it is looking at something even lighter than your subject, so that the meter's attempt to make it a middle tone will result in the right tone. The same is true for dark objects. You must tell your meter that the object is darker than it really is so that the meter's compensation will register the correct tone in the photograph.
You can tell this white (or black) lie to your meter in a couple of ways. You can meter a middle-toned area in the same light as your subject and manually set your camera accordingly. You can lie to the camera outright by telling it that it is using film of an ISO rating different from what is actually in the camera. For instance, if you are using ISO 100 film and are photographing a light subject, you would tell the meter that you have ISO 50 film. If you are photographing a dark object, tell your meter that you have ISO 200 film in the camera. This method works well for cameras in automatic exposure modes. Be careful to tell your camera the truth if you move to a middle toned subject.
Another way to get the right exposure from a lying meter is to politely ignore it. Switch from automatic to manual exposure (a good idea anyway) and simply move to the next larger aperture or the next slower shutter speed (for a light object) or to the next smaller aperture or faster shutter speed (for a dark object).
So it goes. Your meter deceives you, so you deceive it. But what kind of relationship is this? The Ultimate Exposure Computer does not make erroneous assumptions, because you tell it what type of light your subject is in. As long as you tell it the truth, it will not lie to you. Definitely a better relationship.
There is a chance that the preceding section may have confused you. If that is true, don't worry about it. The next section defines Exposure Value, film speed, shutter speed and aperture (f/stop) and describes the interrelationship among these factors. After you've read the next section, revisit this section again, because the concept is important.

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EXPOSURE VALUEFILM SPEED, SHUTTER SPEED AND f/stops -- What Are They?

Each of these four factors is represented by a series of numbers. Your camera may have only a portion of each series indicated.

Exposure Value:

In most light meters, photons of light that are reflected from your subject put pressure on a photo-sensitive receptor in your light meter and are converted into electricity (this is why some meters do not require batteries). The greater the number of photons, the greater the electricity that is produced. Internationally accepted standards specify exactly how much light pressure equals a certain EV number. This value is measured by the following number series (for ISO 100 film):

1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23

Easy! However, even though this appears to be a linear progression, each number represents twice as much light as the numbers increase. This provides an easily memorized scale to clarify the concept of "absolute value of the amount of light falling on a subject". This term is related to illuminance, footcandles and lumens. But it doesn't matter. If you are using a light meter this number is irrelevant, and you can ignore it. You may never see these numbers, because the meter uses them internally to calculate a set of combinations of shutter speeds and apertures. However, knowledge of Exposure Value can significantly enhance your ability as a photographer. The Ultimate Exposure Computer gives you the knowledge to take control of your photographs. See the section "Throw Away Your Light Meter!" for details.
In order to be useful, this Exposure Value number must be converted into an f/stop and shutter speedcombination that will provide the proper exposure for the film speed you have chosen. The relationship between these four elements is represented in the Ultimate Exposure Computer.

Film Speed:

Film speed is a number that indicates the sensitivity of film to light. Film sensitivity is measured by a set of standards established by the American Standards Association (ASA) or the International Standards Organization (ISO). For all practical purposes, the ASA and ISO numbers are interchangeable. In general, films with a higher sensitivity (larger number) have coarser grain and do not register detail as well as films with lower sensitivity (lower number). The number series for film speed is:

25, 50, 100, 200, 400, 800, 1600, 3200

Pretty straight forward! Moving to the right, each number is twice the preceding number, and represents twice the sensitivity to light as the preceding number. There may be some intermediate steps (such as 64 or 125) on your dial. Set the light meter or camera for the same number that is on the film. Your camera may do this automatically.
This leaves only two things to adjust to achieve the correct exposure while making a photograph; shutter speed and aperture (f-stops). Shutter speed and aperture are very important to the creative photographic process.

Shutter Speed:

Shutter speed indicates how long the camera shutter remains open to let light onto the film. The number series for shutter speed is:

15, 8, 4, 2, 1, 2, 4, 8, 15, 30, 60, 125, 250, 500, 1000, 2000, 4000, 8000

This looks more complicated, but it's actually straightforward. These numbers are whole seconds or fractions of seconds. They aren't expressed on your shutter speed dial as fractions to save space, so they should read as below:

15, 8, 4, 2, 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, 1/4000, 1/8000

Again, each number moving to the right is half the value of the preceding number, and represents half as much light as the preceding number.

There is an important rule regarding shutter speeds:

If your shutter speed is slower than the reciprocal of the focal length of your lens, you must use a tripod.

For example, if you are using a 200 mm lens, your shutter speed must exceed 1/200 second (I.E. 
1/250 or faster). If not, you should use a tripod. If your subject is moving, double this shutter speed. If you are moving (such as in a boat or plane) triple the speed. If you are doing macro work (.25 magnification or greater), always use a tripod. If you're using a format larger than 35mm, use a tripod. To be safe, weld your camera to the tripod. If you are a serious nature photographer you will always use a tripod, anyway. A tripod will allow you to make photographs with slower film speeds. Slower film speeds equate to better detail and sharpness in your photographs. More important, using a tripod makes you slow down and allows you to examine your composition more carefully. Modern electronic cameras may lack certain features of the older, mechanical cameras (such as a button that allows you to see the depth of field that will be present in your photograph), but every camera has a tripod socket. You'll make better photographs if you use it.

Aperture (f-stops):

Aperture refers to the size of the opening inside the lens that the light must go through to reach the film. Aperture is measured in f/stops as indicated in the series below:
1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45
This looks tougher, but the solution is the same as for the shutter speeds. These are actually fractions. They should read as follows:

1/1, 1/1.4, 1/2, 1/2.8, 1/4, 1/5.6, 1/8, 1/11, 1/16, 1/22, 1/32, 1/45

Like the shutter speed series, each progression represents half as much light (moving to the right) as the preceding number. But why do the numbers look so odd? I recommend that you ignore it, because it doesn't matter anyway. Just accept that each progression represents half as much light as the preceding number.
If you are curious as to how we got to such a seemingly illogical progression of numbers, read the following three paragraphs. If you aren't curious, you can safely skip them.
The numbers represent the ratio of the focal length of the lens to the diameter of the lens diaphragm opening. That's why it's called an f(ocal)/number. The designation "f/2" means that the diameter of the aperture is 1/2 the focal length of the lens. The designation f/32 means that the diameter of the aperture is 1/32 the focal length of the lens. f/2 on a 100mm lens means that the diameter of the diaphragm opening is 100/2, or 50mm. Unfortunately, the amount of light reaching the film is dependent on the SURFACE AREA of the opening NOT the DIAMETER. As you no doubt learned in high school, the method of calculating the surface area of a circle is Pi times the radius, squared (Pi is approximately 3.14; the radius is half the diameter, squared means that the number is multiplied by itself). Therefore, in our example, the surface area of the opening would be 3.14X25X25, or approximately 2000 sq. mm.
Now, let's look at the next f/stop, which is f/2.8. 100/2.8=35.7mm. The surface area would be 3.14X17.85X17.85. If you multiply it out, you will see that the surface area is now approximately 1000 sq. mm, or HALF the surface area of f/2.Therefore, each succeeding smaller aperture lets in half as much light as the previous f/stop.
The reason we use the ratios instead of the actual surface area of the diaphragm opening is that the actual surface area would be quite different between lenses of different focal lengths. Photographers would have to memorize a series of numbers for each focal length lens they owned. Think about what you'd have to do with a zoom lens! Ratios allow us to use the same number series for all of our lenses. f/2 on a 100mm lens lets in exactly the same amount of light as f/2 on a 500mm lens. Trust me, f/numbers are the best way to do this. I would still recommend that you simply accept that each successive f/stop represents a doubling (or halving) of light. However, since you've read this far, I'll give you a hint about how to memorize the f/number series. The good news is that you only have to remember two numbers; 1 and 1.4. Double each one alternately and you will have two series:

1, 2, 4, 8, 16, 32 and 1.4, 2.8, 5.6, 11, 22

So, combining the two sequences in numerical order gives the full sequence: 

1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32

What you need to remember is that, within each factor, as you move from one number to the next, the amount of light is either doubled or halved.
Some cameras may include numbers between the numbers mentioned above. Those numbers represent half or third stops. For the purpose of what you are learning here, ignore the in-between numbers.

HOW TO USE THE ULTIMATE EXPOSURE COMPUTERTHROW AWAY YOUR LIGHT METER!

Well..... maybe not. But you could if you wanted to. The purpose of this section is to show you how Exposure Value, Film Speed, Aperture and Shutter Speed are interrelated, and why you don't need a light meter to know the proper exposure.

STEP ONE: PUT YOUR CAMERA ON MANUAL

Automatic cameras are designed to give an average photographer a better chance of getting properly exposed pictures while shooting average subjects in average light, without a tripod to steady the image. These cameras work well for family pictures and many urban subjects. Automatic focus and exposure (especially TTL flash) are extremely useful for some specialized types of photography, such as sports or photojournalism.
Nature photography is a different matter. Many of your subjects will not have "average" tonality. Most of your subjects will not be located in "average" light. Many times your subject will not be in the center of the frame (where an automatic camera takes its light readings). However, the most important reasons for putting your camera on manual while photographing natural subjects are:
1) There are times when you will want to stop motion, such as a flower waving in the breeze. Other times you may want to exaggerate the effect of motion, such as when photographing flowing water. Decisions about these matters address APPARENT SUBJECT MOTION. This is controlled by the shutter speed. Faster shutter speeds stop action better than slower speeds (1/1000 second will stop most motion, while 1/8 second will blur even slow moving subjects).
2) Sometimes you will want everything in focus, such as a foreground flower AND a distant mountain. Sometimes you will want the flower to be in focus while keeping a busy background softly out of focus. When you make this type of decision, you are considering DEPTH OF FIELD (the distance in front and behind of the subject that is acceptably in focus). The aperture (f/stop) controls the depth of the image that is acceptably in focus (f/32 keeps more of the subject in focus than f/2).
You must make these aesthetic choices. The camera has no way of knowing what you want the photograph to look like. Remember that the camera's automation is designed to make technical decisions based on average subjects in average light. It is not designed to make aesthetic choices regarding natural subjects (which are rarely "average").
Because of these factors, the same camera that will give you dozens of perfect photographs of your children opening birthday presents will churn out dozens of badly exposed nature photographs that look nothing like what you experienced in the field. The only way for you to get good results in nature photography is for YOU to make the choices.
Get your camera. Purchase two sets of batteries and install one set in the camera. Do this even if you changed batteries last month. Keep the spare set with the camera, always. Keep a copy of the Ultimate Exposure Computer with the camera and the batteries.
Find your instruction manual. Now comes the hardest part of the entire process (unless you've lost the manual). You must find out how to set your camera body to its "manual" setting. I can't help you here. Good luck and congratulations! You've just taken the first step toward becoming a serious nature photographer!

STEP TWO: A TOUR AROUND THE ULTIMATE EXPOSURE COMPUTER

CHART A: Take a look at Exposure Value Chart A. The left column of the chart contains the Exposure Value (EV) numbers. The right column of the chart contains some lighting situations that equate to EV numbers. The chart ranges from EV -6 to EV 23. This represents a range of light from below reflected starlight to the brightest light in the Solar System.
Notice that most daylit subjects fall within a narrow range from EV 11 to EV 15. It is a good idea to memorize the characteristics of these daylight Exposure Value factors. Pick a favorite lighting situation, note the EV and turn theUltimate Exposure Computer over to Chart B.
CHART B: Exposure Factor Relationship Chart B shows how Exposure Value, film speed, aperture and shutter speed are related.
On the left side of this chart, along the top row, are the ISO/ASA film speed numbers. Pick the film speed you use most often. Starting with that film speed, move down the column until you reach the EV number that you chose from Chart A. Now move to the right along the row until you cross the double line. To the right of the double line is a row of shutter speeds. As you will recall, a fast shutter speed stops motion, while a slow shutter speed enhances the effect of motion. Choose an appropriate shutter speed for your subject. Now, move straight up the column to the top row to find the aperture (f/stop) for use with the shutter speed. Enter your chosen shutter speed on your camera and set your lens to your chosen f/stop. It's that easy!


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Example 1 -- A Typical Day In The Life of a Nature Photographer

Suppose you are in the field photographing with ISO 100 film, and the battery in your camera just failed. You can't take any light readings, and you only have one mechanical shutter speed (1/125 second). This situation is extremely unfortunate, since you just stumbled onto an extremely rare species of flowering plant that blooms for one hour every hundred years. It's in full bloom. The sky has started to cloud over and things look bleak. Have no fear! Your Ultimate Exposure Computer will save the day! You already know from Chart A that subjects under heavy cloud cover are at EV 12.
Look on the left-hand side of Chart B. Locate ISO 100 film along the top row. Look down the column until you find EV 12. Look along the row to the right of EV 12 (across the double line) until you find the shutter speed you are looking for (your mechanical 1/125 second). Move straight up the column and find that the aperture of your lens should be set to f/5.6. Easy! No light meter required!

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But wait! Suppose the f/stop you landed on isn't on your lens. Or perhaps the f/stop you landed on offers too much or too little depth of field (the distance in front and behind of the subject that is acceptably in focus). In that case, move to an appropriate f/stop along the top row to achieve your desired result. Then move down the column to the row represented by your original choice of film speed and EV number. At that intersection you will find the appropriate shutter speed for the desired aperture.
Perhaps this exercise indicates a very slow shutter speed that cannot be hand held. That means one of two things. You must put the camera on a tripod (where it should have been to begin with), or you need to determine the proper film speed to use for your photographic situation.


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Example 2 -- Planning Ahead

The Ultimate Exposure Computer is useful for several photographic tasks, such as planning a shooting session. For instance, you may know that you are going to be shooting scenics of a field of flowers with mountains behind. It will be a sunny day (EV 15), but the wind will be gusting to twenty miles per hour. You need a small aperture to get the greatest depth of field (the distance in front and behind of the subject that is acceptably in focus), but you will also need a fast shutter speed to stop the motion of the flowers in the wind. What ISO film speed will you need to do the trick?
Go to the upper right of the Table B. Find f/22 aperture for maximum depth of field. Go down the column until you reach a shutter speed fast enough to handle the flowers in the wind (1/125 second). Move to the left along the row (across the double line) until you find the correct Exposure Value (15). From there, move up the column and find that ISO 200 speed film will solve your problem.


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STEP THREE: USE THE ULTIMATE EXPOSURE COMPUTER WISELY

The Ultimate Exposure Computer is designed for use in existing light. If you need to use flash, or you are taking bellows shots, by all means use a camera with a through-the-lens meter. When using flash, use the manufacturer's dedicated flash unit that allows through the lens control of the flash. Those situations are excellent reasons to use the meter in your camera. Some of the lighting situations described in Chart A occur outdoors during daylight. In these cases it is assumed that the sun is at an elevation greater than forty-five degrees. If the sun's elevation is less than forty-five degrees, you can use the next lower Exposure Value, although you'll probably like the results of using the chart without modification. For some films, exposures involving shutter speeds in excess of several seconds may require additional exposure because the film's sensitivity decreases with continued exposure to light for long periods (this is called "reciprocity failure"). Light meters do not correct for this phenomenon, because it varies according to the type of film. Consult the manufacturers' specifications for details. There are some tricky exposures where you can improperly expose the film whether you are using a camera meter or the Ultimate Exposure Computer. Many of these situations are addressed in "What to do in Tricky Light Situations" in Appendix A.


THE FINAL STEP: THROW AWAY THE ULTIMATE EXPOSURE COMPUTER!

The Ultimate Exposure Computer is easy to use and extremely accurate. But there is an easier way. Why not memorize the whole thing? It's a lot easier than you may think. This section will show you how.

There is a rule called the "Sunny f/16 Rule". It says:

On a bright, sunny day, the correct exposure for any subject is f/16 at the shutter speed nearest to the reciprocal of the film speed.

For example, if you are using ASA/ISO 100 film, the correct exposure would be 1/100 second (or rather the closest available shutter speed - 1/125 second) at f/16. The next three paragraphs address this example.
We know that a bright, sunny day equals EV 15. The "Sunny f/16" rule gives us an anchor point to use in mentally calculating the entire contents of Chart B, since it contains all four elements of exposure: Exposure Value (EV 15), film speed (ISO 100), aperture (f/16) and shutter speed (1/125).
From that anchor point, it is easy to calculate any other set of correct exposures. We know that each step up or down in one variable represents a doubling or halving of the amount of light required to make a correct exposure. For example, an overcast day would halve the light falling on the subject (EV 14). If the light reaching the film is cut in half, ONE other variable needs to be changed to increase (double) the amount of light. In this case the shutter speed could be reduced to 1/60 second OR the aperture could be increased to f/11 OR the film speed could be increased to ISO 200. Any ONE of these corrections would provide the correct amount of light. Make the correction that best suits your photographic purpose (decreased depth of field -- the distance in front and behind of the subject that is acceptably in focus -- or enhanced apparent subject motion).
In the same sense, a change in any of the variables can be offset by a reciprocal change in any other variable. For example, you may need a faster shutter speed to stop some action. If you choose 1/500 second, the light will have been reduced by two steps (cut in half from 1/125 to 1/250 and cut it in half again from 1/250 to 1/500). You could compensate for this by opening up the aperture two steps (from f/16 to f/11 will double the light and f/11 to f/8 will double it again). You could make the same correction by increasing the film speed by two steps (from ISO 100 to ISO 200 doubles the light and from ISO 200 to ISO 400 doubles it again). Or, you could increase the aperture to f/11 AND increase the film speed to ISO 200. Again, make the adjustment that best suits your purpose.
Now, let me say that the "sunny f/16" rule is one of the most misunderstood rules in photography. I have heard very competent professional photographers say that the rule is based on reflected light. Nonsense! The rule is based on the light falling on the subject (incident light) not the light reflected from the subject. The rule gives precisely the same measurement that you would get from an incident meter or a reflected meter using a perfect gray card. If your subject is very light or dark, you will need to adjust exposure to bring it within the five stop range of transparency film. But you would also have to make an adjustment from a reflected reading in the same circumstance. The difference is that an incident light measurement is a consistent anchor from which to adjust. A reflected measurement is much more subjective and prone to error. I have also heard that the "sunny f/16" rule doesn't work for backlit or sidelit subjects. This is also false. Just open up a half stop, and make a series of exposures in half stop increments one and a half stop above and below the correct exposure. This is called "bracketing" and it is an important concept. You should always bracket in difficult situations. Anyone who says differently is misleading you. See the section on "Difficult Exposures" for guidelines.
The "sunny f/16" anchor point, combined with your knowledge that each change of one step in a factor doubles or halves the exposure, makes it easy to select a correct exposure for any photographic situation you may be confronted with.
Practice this technique with the Ultimate Exposure Computer in hand. Look at (or imagine) a photographic situation and try to mentally figure out the correct exposure using the "Sunny f/16" anchor point and any adjustments you feel are necessary. For the purpose of this exercise, it is best to concentrate on EV 11 through 16 (where most of your outdoor photography will occur). As you perfect your accuracy in this range, you can move on to more exotic lighting situations. Write down the factors (Exposure Value, film speed, aperture and shutter speed) that you believe will make the best exposure. Consult the Ultimate Exposure Computer to assess the accuracy of the calculations you made mentally.
With a little effort you will soon be achieving close to 100% accuracy, without the use of a light meter or any other exposure aid. The more you practice with the Ultimate Exposure Computer, the sooner you will be able to simply look at a lighting situation and immediately know the correct exposure for the job.
Once you have mastered this technique, you can use it to judge the accuracy of the light meter in your camera (or anyone else's). You will be able to amuse and edify your photographic friends by accurately stating the correct exposure in any situation before your friends can take a reading with their meter!
When you reach that point, the real Ultimate Exposure Computer will be
YOU!

WHERE ON EARTH ARE YOU?

I get e-mail from all over. Photographers use this document on every continent. Even Antarctica. People taking photographs far to the north or south of my sunny San Diego, California (USA) location (32d N) have asked me to make charts adapted to extreme (from my perspective) latitudes. Here's why: If you live anywhere between the equator and about 50 degrees north or south, the charts and tables in the Ultimate Exposure Computer should work fine for you (remember to bracket). Just move on and don't read the next few paragraphs. If you live further north or south, it should work for you in summer. But things aren't so simple if you live toward the poles. Light from the sun is diminished by traveling farther through the atmosphere or blocked by the curvature of Earth. Look at it this way: near a solstice, natural light at mid-day can range continuously from "Sunny f/16" at the equator to near dark at one of the poles. So you have the variables of latitude and season and time of day to consider. There are other variables such as altitude and snow cover (partially counteracting the effect of latitude and season).
"So," you say, "You've taken me this far and it turns out this won't work for me?"   I say, the Ultimate Exposure Computer will NOT let you down. "Well then," you say, "Are you going to give me some complicated, hashed-up chart that tries to fit in the five variables you talked about?"  Well... Actually I DID try to do this. What a mess! I'm striving for simplicity here, not trying to model the light falling on every square inch of Earth! Fortunately, the solution is simple, because I know something about you that you've never admitted to me. You DIDN'T throw away your light meter, did you!!? Hah! I knew it!  Good! Read on!
You can easily calibrate the Ultimate Exposure Computer to fit your exact location. Go outside and take a meter reading (preferably incident or reflected from a standard gray card). Use whatever light is available, but it is best if it's full sun on a bright day (Sunny f/16). Take a few readings and average them. How many stops difference between your reading and the "Exposure Value Chart"?
Print out two copies of the "Exposure Value Chart" (below). Using a sharp implement (your choice) cut one chart apart, separating the "EV" section from the "Type of Lighting Situation" section. If, for example, your metered reading was two stops below "Sunny f/16" you would slip the "EV" section down two rows. Tape it in place. You now have a chart customized to your latitude, season, time of day, altitude and any other variable you can think of (remember to bracket). You'll need to repeat this exercise as the seasons and conditions change. This is only necessary for sunlit subjects. Other subjects in the chart will work using the copy you didn't deface. There are limitations. Here's just one example: If you're at McMurdo Station on June 21, you'll need to find another way to calculate exposure. Drop me an e-mail and tell me how you make photographs. For most of you intrepid poleward photographers. enjoy!

Exposure Value Chart


EV

TYPE OF LIGHTING SITUATION

-6
Night, away from city lights, subject under starlight only.

-5
Night, away from city lights, subject under crescent moon.

-4
Night, away from city lights, subject under half moon. Meteors (during showers, with time exposure).

-3
Night, away from city lights, subject under full moon.

-2
Night, away from city lights, snowscape under full moon.

-1
Subjects lit by dim ambient artificial light.

0
Subjects lit by dim ambient artificial light.

1
Distant view of lighted skyline.

2
Lightning (with time exposure). Total eclipse of moon.

3
Fireworks (with time exposure).

4
Candle lit close-ups. Christmas lights, floodlit buildings, fountains, and monuments. Subjects under bright street lamps.

5
Night home interiors, average light. School or church auditoriums. Subjects lit by campfires or bonfires.

6
Brightly lit home interiors at night. Fairs, amusement parks.

7
Bottom of rainforest canopy. Brightly lighted nighttime streets. Indoor sports. Stage shows, circuses.

8
Las Vegas or Times Square at night. Store windows. Campfires, bonfires, burning buildings. Ice shows, football, baseball etc. at night. Interiors with bright florescent lights.

9
Landscapes, city skylines 10 minutes after sunset. Neon lights, spotlighted subjects.

10
Landscapes and skylines immediately after sunset. Crescent moon (long lens).

11
Sunsets. Subjects in deep shade.

12
Half moon (long lens). Subject in open shade or heavy overcast.

13
Gibbous moon (long lens). Subjects in cloudy-bright light (no shadows).

14
Full moon (long lens). Subjects in weak, hazy sun.

15
Subjects in bright or hazy sun (Sunny f/16 rule).

16
Subjects in bright daylight on sand or snow.

17
Rarely encountered in nature. Some man made lighting.

18
Rarely encountered in nature. Some man made lighting.

19
Rarely encountered in nature. Some man made lighting.

20
Rarely encountered in nature. Some man made lighting.

21
Rarely encountered in nature. Some man made lighting.

22
Extremely bright. Rarely encountered in nature.

23
Extremely bright. Rarely encountered in nature.

THE ULTIMATE EXPOSURE COMPUTER
EXPOSURE VALUE

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Exposure Factor Relationship Chart B

FILM SPEED (ISO/ASA NUMBER)

APERTURE OF LENS (f/STOP)

ISO 25

ISO 50

ISO 100

ISO 200

ISO 400

ISO 800

ISO 1600

ISO 3200

f/1.4

f/2.0

f/2.8

f/4

f/5.6

f/8

f/11

f/16

f/22

f/32
10-1-2-3-4-5-64 sec8 sec15 sec30 sec1 min2 min4 min8 min16 min32 min
210-1-2-3-4-52 sec4 sec8 sec15 sec30 sec1 min2 min4 min8 min16 min
3210-1-2-3-41 sec2 sec4 sec8 sec15 sec30 sec1 min2 min4 min8 min
43210-1-2-31/2 sec1 sec2 sec4 sec8 sec15 sec30 sec1 min2 min4 min
543210-1-21/4 sec1/2 sec1 sec2 sec4 sec8 sec15 sec30 sec1 min2 min


E
6543210-11/8 sec1/4 sec1/2 sec1 sec2 sec4 sec8 sec15 sec30 sec1 min
S

V
765432101/15 sec1/8 sec1/4 sec1/2 sec1 sec2 sec4 sec8 sec15 sec30 sec
H
876543211/30 sec1/15 sec1/8 sec1/4 sec1/2 sec1 sec2 sec4 sec8 sec15 sec
U

N
987654321/60 sec1/30 sec1/15 sec1/8 sec1/4 sec1/2 sec1 sec2 sec4 sec8 sec
T

U
1098765431/125 sec1/60 sec1/30 sec1/15 sec1/8 sec1/4 sec1/2 sec1 sec2 sec4 sec
T

M
11109876541/250 sec1/125 sec1/60 sec1/30 sec1/15 sec1/8 sec1/4 sec1/2 sec1 sec2 sec
E

B
121110987651/500 sec1/250 sec1/125 sec1/60 sec1/30 sec1/15 sec1/8 sec1/4 sec1/2 sec1 sec
R

E
1312111098761/1000 sec1/500 sec1/250 sec1/125 sec1/60 sec1/30 sec1/15 sec1/8 sec1/4 sec1/2 sec

R
14131211109871/2000 sec1/1000 sec1/500 sec1/250 sec1/125 sec1/60 sec1/30 sec1/15 sec1/8 sec1/4 sec
S

S
151413121110981/4000 sec1/2000 sec1/1000 sec1/500 sec1/250 sec1/125 sec1/60 sec1/30 sec1/15 sec1/8 sec
P
1615141312111091/8000 sec1/4000 sec1/2000 sec1/1000 sec1/500 sec1/250 sec1/125 sec1/60 sec1/30 sec1/15 sec
E
17161514131211101/15000 sec1/8000 sec1/4000 sec1/2000 sec1/1000 sec1/500 sec1/250 sec1/125 sec1/60 sec1/30 sec
E
18171615141312111/30000 sec1/15000 sec1/8000 sec1/4000 sec1/2000 sec1/1000 sec1/500 sec1/250 sec1/125 sec1/60 sec
D
19181716151413121/60000 sec1/30000 sec1/15000 sec1/8000 sec1/4000 sec1/2000 sec1/1000 sec1/500 sec1/250 sec1/125 sec
S
20191817161514131/125000 sec1/60000 sec1/30000 sec1/15000 sec1/8000 sec1/4000 sec1/2000 sec1/1000 sec1/500 sec1/250 sec
21201918171615141/250000 sec1/125000 sec1/60000 sec1/30000 sec1/15000 sec1/8000 sec1/4000 sec1/2000 sec1/1000 sec1/500 sec
22212019181716151/500000 sec1/250000 sec1/125000 sec1/60000 sec1/30000 sec1/15000 sec1/8000 sec1/4000 sec1/2000 sec1/1000 sec
23222120191817161/1000000 sec1/500000 sec1/250000 sec1/125000 sec1/60000 sec1/30000 sec1/15000sec1/8000 sec1/4000 sec1/2000 sec

THE ULTIMATE EXPOSURE COMPUTER
EXPOSURE FACTOR RELATIONSHIP CHART B ©1995-2013 Fred Parker

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Some Useful Photographic Guidelines

Sunny f/16 Anchor Point: On a bright day (EV 15) the correct exposure for any subject is f/16 at a shutter speed nearest to the reciprocal of the film speed (I.E. ISO 100 film = 1/125).
Doubling Rule: Within any exposure factor (Exposure Value, film speed, aperture, shutter speed) each step is double (or half of) the preceding step.
Tripod Rule: Use one for every photograph. If your shutter speed is slower than the reciprocal of the focal length of your lens (I.E.< 1/200 sec. with a 200 mm lens), use a tripod. If your subject is moving, double this shutter speed. If you are moving (such as in a boat or plane) triple the speed. If you are doing macro work (.25 magnification or greater), always use a tripod. If you're using a format larger than 35mm, use a tripod. To be safe, weld your camera to the tripod. 
Film Speed: As you increase film speed, problems with grain and sharpness will increase.
Apparent Subject Motion: Increasing shutter speed will reduce apparent subject motion. Decreasing shutter speed will increase apparent subject motion.
Depth of Field (the distance in front and behind of the subject that is acceptably in focus): Increasing aperture (lower f/stop #) will decrease depth of field. Decreasing aperture (higher f/stop #) will increase depth of field.

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APPENDIX "A": WHAT TO DO IN TRICKY LIGHT SITUATIONS 

Front Lighting: Follow directions given in this document.
Side Lighting: Expose for front lighting, exactly as described herein. Bracket in half stop increments to one and a half stops over exposure. No need to bracket on the underexposure side. Alternatively, use a reflecting surface to bounce more light into the shadows. Make decisions based on the tonality of the subject. With a light subject your first exposure should be your best. With a darker subject your last exposure should be correct.
Back Lighting: Where possible, shoot subject in front of a dark background. Use EV 12 as your starting point (there are three stops difference between sun and shade) and bracket in half stop increments one and a half stops to the underexposure side (no need to bracket to the over exposure side -- EV12 takes care of it for you). If you must shoot into a light background, your best bet is to shoot a silhouette. Examples are backlit fog on a lake with fishermen in boats in the shot. In this case, start with EV 15 and bracket in half stop increments to three stops on the under exposure side. Your backlit photography will be better if you use a long lens (with an appropriate lens shade!). Make sure your lens is scrupulously clean. If you are shooting people, use a warming filter (such as 81B), and adjust exposure by increasing exposure by 1/2 to one stop.
The Ultimate Backlight -- Sunrises & Sunsets: When the sun is in your shot, go for silhouettes. Begin with EV 15 and bracket in full stops to four stops under.
Night Shots: Don't shoot them at night, shoot at dusk. Look through your camera with the aperture fully stopped down. This has the effect of taking color out of the scene and allowing you to judge tonality more accurately. When your subject and the sky seem to be the same shade of gray, open up the lens and make your exposure. With a middle tone subject, this will usually take place 30-45 minutes after sundown. Use EV9 as a basis for exposure. Bracket in half stop increments to two stops over and two stops under. You will like at least half of the shots.
Shoot the Moon With a Long Lens: Use EV14. Bracket in half stop increments to one stop over and one stop under.
Shooting Scenics in Woods or Where There Are A Lot of Shadows: Shoot under overcast (EV12 or 13) situations.
Fast Moving Subjects -- Sports, Etc.: Pick a vantage point that lets you shoot in front light, if possible. Utilize a dark background if available. Use a telephoto lens and a tripod, if possible. Use Chart B to select a film speed that will allow you to use a shutter speed TWICE the reciprocal of your focal length (i.e. a 300mm lens at 1/500 sec).
Shooting from an Airplane or Boat: Use Chart B to find a film speed that will let you shoot at THREE times the reciprocal of the focal length (i.e. 1/1000 sec for a 300mm lens).
Macro or Micro Shots: Make adjustments for loss of light due to extension of the lens or bellows, as given in the instruction manual, or on the lens barrel. Better yet, use your in camera meter! It will need the exercise.
Excessively Long Exposure Times: Times above 10 seconds or so. Here you run into reciprocity failure (a fancy name for "it will take a LOT longer than you think!") Follow the directions that come with every film package. You will run into this problem whether you are using a meter or not. Bracket liberally!


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APPENDIX "B": EV, FOOTCANDLES AND LUX (for REALLY serious light aficionados).

Those of you who have been involved with photography for some time may have heard the terms "Foot-candles" or "LUX". You may find these referred to on some light meters. These measurements are used in the film industry, television and industrial lighting. There is a relationship between these numbers and EV numbers. Since you've already learned about EV numbers, this section will allow you to edify and amaze your friends with your knowledge of light!

RELATIONSHIP

Is there any intuitive relationship between EV, Foot-candles and LUX? The answer is "not really". EV numbers are a linear progression that is convenient to use for memorization of relative light levels. Footcandles and LUX proceed in a doubling manner, which better illustrates the doubling (or halving) of light at each step. To get to LUX, simply multiply the foot-candles by 11. The table below better represents the relationship:




Exposure Value
EV ISO 100

Footcandles

LUX

-3

.03

.31

-2

.06

.63

-1

.12

1.3

0

.23

2.5

1

.46

5

2

.93

10

3

1.9

20

4

3.7

40

5

7.4

80

6

15

160

7

30

320

8

60

640

9

120

1300

10

240

2600

11

480

5100

12

950

10000

13

1900

20000

14

3800

41000

15

7600

82000

16

15000

160000

17

30000

330000