Exposure : Daniel Nielsen Photography

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Last Updated: February 2, 2004

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Exposure

By Daniel Nielsen

SYNOPSIS: All exposure meters assume the scene before them averages 18 percent gray in tones. That is true more often than not, so your meter usually does just fine. If the scene doesn't average 18 percent gray, your meter will expose it incorrectly. You can use your head to adjust exposure so it is correct. You can even use your head to set the correct exposure without consulting a meter at all. There is no one "correct" exposure for a particular scene; rather, several different exposures can represent the scene in different ways.

FILM

If you aren't happy with the exposure quality you get most of the time with print film, shoot a roll of slide film. You will be amazed at the instant correlation between how you set the exposure and how the slide turns out. That's because slides are a one-step medium - the piece of film in your camera is the same piece of film that is the final photograph. As long as it is processed properly (and today's labs are quite reliable), there are no variables after you expose it. The print film process, however, inserts either a machine or another human into the process between you and the final print - that extra step makes it difficult to determine the quality of the actual exposure you made. Slide film has less latitude than print film, so it may not be the film of choice for your style. But there's no substitute for it when you want to check how accurate your in-camera exposures are. As a bonus, color, focus and sharpness all are much more easily seen in a slide than in a negative.

DETERMINING EXPOSURE

With today's built-in meters, many amateur photographers never give a thought to exposure. And a good percentage of the time, they get exactly the exposure they desire. But once in a while, they see a great shot in their viewfinder and are disappointed with what shows up in the final photograph. That's because even the best meters can't do what your brain can do: Look at a scene and decide what is the most aesthetically important part of the subject and decide how that should appear in the final photograph. Yes, your brain can do that. Here are some rules, some good and some bad, that purport to have bearing on exposure.

SUNNY 16 RULE

For starters, try taking a whole roll of film without using your meter at all! To do that, use the old "Sunny 16 Rule". When it's sunny out and your subject is in full sun, set the aperture at f16 and the shutter speed at the inverse of your film ASA. If you've loaded up with 100 ASA film, set your shutter speed at 1/125; 400 ASA film requires a shutter speed of 1/500. Just choose the shutter speed closest to your ASA. The result will be properly exposed film - compare it to your meter if you must, and you'll find the settings generally agree.

• If there are light clouds covering the sun, open up one stop to f11.
• If clouds are covering the sun (cloudy bright), open up two stops to f8.
• On a dark, overcast day, open up to f4.
• If it's a bright sunny day but your subject is in full shade, use f4.

When you get your prints or slides back from the processor, you will be pleasantly surprised that most of your exposures will be right on the money.

Of course, if you move indoors or wait until twilight, judgement becomes much more difficult, and then a meter is the easiest way to attain proper exposure. But you needn't be totally dependant on a meter. Use your meter to help you learn to judge exposures with your eye - take note of what the meter readings are in various lighting siutations, and memorize a few basic settings. You'll soon develop a feel for what exposures should be. Fred Parker goes into great depth on how to learn this and how use your head to get more accurate exposures than your meter can.

Here are a couple more basic exposure suggestions, based on a film speed of 100 ASA [if you're using 400 ASA film, you can close down two stops or, alternatively, use a shutter speed two steps faster]:

• A brightly lit city street at night, f2 at 1/30 (or f4 at 1/8).

• The moon at night, f8 at 1/250. [Unless you use a spot meter, a reflected light meter will be wrong, because it will read mostly the black sky, badly overexposing the moon itself.]

• If you are photographing a subject backlit (with the sunlight coming from behind the subject), open up two stops from your camera's meter reading. If you don't, the subject will appear in the photograph as a silhouette.

BRACKETING

Since neither meters nor estimating exposure will always be exactly right for the effect you want to create, it is good practice to take a series of pictures with different exposure settings. Pros do it all the time. Of course, if the moment is fleeting, you take your one shot and hope for the best. But if you subject matter allows, take three or five pictures that are identical except for exposure - if you've decided on a shutter speed of 1/250th of a second, take frames at the meter's recommendation, for example, f.5.6, then take additional frames at f4 and f8. Some pros bracket in half-stop increments, so they would add to more frames in between those three. If the lighting conditions are really unsual, you might widen the range and shoot more frames at f2.8 and f11. Frequently, two or three of the resulting exposures will be acceptable, but will each convey a diffferent feeling or atmosphere. If you are shooting slide film and run across a particularly great image, shoot several frames just to ensure you have duplicates in case one is damaged or lost in the future (another frame shot on the spot is much cheaper than paying for a lab to make duplicates in the future). Though varying the amount of light is the most common use of bracketing, it can be useful in other ways, too.

KEEP THE SUN AT YOUR BACK (wrong!)

This rule has been packaged with simple snapshot cameras for a century. It is aimed at ensuring there is plenty of bright light on your subject. It also tends to minimize flare. But if you always keep the sun at your back when shooting, you will never have shadows in your pictures. Shadow and light, and the contrasts between them, are really the subject of all photography. So if you always keep the sun at your back, you will deny yourself many chances to make visually pleasing photographs.

F8 AND BE THERE

• This is not an exposure rule. But you hear it mentioned frequently in relation to exposure, and it is rarely explained. So here's the scoop.
"F8 and be there" is an old photojournalist's adage. I haven't been able to pin down who coined the term, but it was in common use back in the 1970s when I was getting started in newspapers. It means: Set your aperture at f8 so you'll have useful depth of field but will still be able to use a reasonable shutter speed in most situations, then be in the right place at the right time. If you're already at f8 (which also is an aperture that produces optimum sharpness for most lenses), you need only adjust your shutter speed, compose your picture and press the button. "f8 and be there" basically means think ahead, have your equipment at the ready, and put yourself where you can get the best angle. It is not an exposure rule, but a reminder that 90 percent of photojournalism is being in the right place at the right time. The f8 component of "f8 and be there" has largely been replaced in photojournalism by auto program exposure modes.

EQUIVALENT EXPOSURES

For any lighting situation, there are many combinations of aperture and shutter speed that will result in the identical amount of light reaching the film.
1/500 at f4 1/250 at f5.6 1/125 at f8 1/60 at f11 1/30 at f16 1/15 at f22 1/8 at f32
All of the above are the identical when it comes to total amount of light reaching the film. Of course, depth of field changes with each aperture change. And shutter speed changes affect camera shake and how motion is recorded on the film.

All of the above assumes you're using ambient light. Everything changes if you throw a flash unit into the equation - but that's another article.

18 PERCENT GRAY

All light meters always assume they are pointed at a scene that averages out to 18 percent gray (except modern matrix meters - see below). Why 18 percent? Because many scenes in nature average out to -- you got it -- 18 percent gray.

Handheld meters and many older in-camera meters measure the entire image area equally - that's called averaging metering. In the 1960s, Nikon began using center-weighted metering, in which the center of the image area is given more weight (importance) in deciding what the light level is. That makes sense, since the subject in most photographs is in the center area of the frame. This scheme also reduces the effect of bright sky that typically is in the top edge of the frame in outdoor shots. Most other manufacturers followed suit and switched to center-weighted metering.

This concept of 18 percent gray is why incident light meters work. This kind of handheld meter has a dome, cone or disk of translucent plastic that is simply held in light similar to the light the subject is in. If you're taking a photo of a friend who is sitting in the shade of a tree 50 feet away with a normal 50mm lens, and that tree is in the middle of a bright sunlight wheat, your camera meter will see way too much bright wheat and sky, and your friend will be dreadfully underexposed (will appear dark in the final photograph). With an incident meter, you can hold up the meter, use your hand of body to shade it (since the subject is in the shade), and come up with the correct exposure.

If you don't want to buy an incident meter, you can accomplish the same feat with a $5 gray card. That's simply a piece of cardboard printed with an 18 percent gray tone. To use it, hold it in light similar to your subject, then point your camera lens at it close enough to fill the frame. Take your meter reading. If you don't have a gray card, use the palm of your hand but then open up one stop (your palm is about one stop lighter than 18 percent). 18% gray reflects five times less light than white; so you could use a piece of white card instead of 18% gray, then close down two and a third stops [Remember, each stop you close down cuts the amount of light in half. Therefore, closing down two stops lets in a quarter the amount of light - then add a third of a stop for that fifth factor of difference. In reality, two stops will normally work fine. But in dim light, bracketing is a very good idea.] In fact, this method can be used in dim lighting conditions where your meter can't detect the light level reflected from a gray card, but can detect the light reflected from the brighter white card.

Spot meters also assume they are looking at a subject that is 18% gray. But they're designed to "see" only a very small "spot," typically an angle of one degree or four degrees. They are used by pointing them (they usually have a viewfinder with a center spot to defines the sensitive area) at a small portion of the scene and metering only that tiny spot. If you point it at a spot that you want rendered as 18% percent gray, you're all set.

Nikon and other camera makers today use a refinement of center weighting sometimes called evaluative metering. In this system, the meter has several areas within the frame of varying sensitivity, and the in-camera computer reads the light levels in each of the separate areas, averages them (unless it finds one area that is wildly different than the others, in which case it assumes that is a light source or bright reflection and basically tosses out that area's reading) and sets the overall exposure.

Nikon's matrix meter was the first system that went beyond the simple 18% gray rule. Canon later followed suit. The matrix system's computer is designed to be aware of the sunny 16 rule. That is, if a matrix sensor detects a light level higher than sunny 16, it knows that what it's reading can't be an average scene. If the high level of brightness is confined to one segment, the meter ignores that segment, assuming it is looking at a bright reflection, which should be rendered as white, not 18% gray. If most of the segments read higher than sunny 16, the matrix computer knows that the scene most likely is lighter than average, perhaps snow or light sand. As a result, it increases the exposure in an attempt to appropriately compensate (to the degree the reading is above sunny 16). Generally, the system works very well. But it's not perfect, and there are situations where even matrix or evaluative meters can be fooled. Through experience (looking at your slides), you can learn in what kinds of lighting situations your camera's meter tends to err.

CONTRAST

Perhaps the final piece of the basic exposure puzzle is contrast. This is the range of light values, from bright to dark. Film can record a much smaller brightness range than your eyes can see. If you study your favorite landscape photos, you'll probably notice most or all of them were made near dawn or dusk. It's not coincidence that those are the times of day when contrast is relatively low, well within the variation film can record. When you take a photo at noon on a sunny day, the contrast range is extremely high - and that's why you get black spots where faces should be and white blobs where frothy whitecaps should be. The contrast is just too much for film.

One solution is to emulate the pros and avoid shooting during the midday hours. A great side effect of this is that most interesting light - shadows, orange sunsets, etc., occurs early and late in the day. Interesting light also appears in stormy weather, when contrast again is relatively low. Another solution, useful only with fairly near subjects, is to use fill flash to add light to the shadowed areas of the subject, thereby lowering overall contrast.

When I'm on vacation, I take photos at noon just like every other tourist. I rarely use daylight fill flash. I just do my best to find lower contrast subjects, or make use of high contrast the best I can.

YOU'RE SMARTER THAN YOUR CAMERA'S METER

Incident meters and center-weighted meters work great. Most of the time. But what if you don't want your subject reproduced as 18 percent gray (which is what all of the above methods aim for)? Matrix meters are much more likely to come up with a correct exposure in contrasty situations than are the older center-weighted meters. But they're not perfect, and you are still smarter than the meter.

For example, what if you're shooting a coal-black locomotive and want it to come out black in the final print? Or what if you're photographing the fine lace that is part of a bride's white dress, and you want it to appear white in the resulting slide? To make any of those things happen, you need take a meter reading, then improve on it.

In the case of the wedding dress, your meter will do its duty and make it turn out 18 percent gray. Knowing that, you need to open up an extra stop, from, say f8 to f5.6, to make the lace turn out white in your photograph. Same thing if you're shooting a scenic of a snow-covered field or light-colored sand dunes.

In the case of the black locomotive, or any dark-toned subject, you need to close down a stop, for example from the f11 your meter may suggest to f16, if you want the locomotive to appear black instead of 18 percent gray. Again, if you're using matrix metering, you may be safe in letting the system do the thinking for you - or you may get a better exposure by overriding it.

This is process of outthinking your meter is the most basic incarnation of the famed Zone System created by Ansel Adams. (The full-fledged Zone System is a complicated combination of metering technique and development adjustment designed to control exposure values and contrast ratios. In its full form, it is useable only with sheet film, where each exposure can be developed differently.) Knowing that your meter tries to make your scene average 18 percent gray in your final print, use your brain to adjust the exposure. Use your meter, then think it one better!

THERE ISN'T ALWAYS A SINGLE "CORRECT" EXPOSURE

When you bracket, you will frequently find two or more different exposures that create different but "correct" results. The difference will be in which portions of the scene are rendered light, medium or dark. The entire Zone System of exposure revolves around visualizing how you want the final picture to look (as far as exposure values) before taking it. Bracketing is, in a way, a shotgun-approach variation of the zone system - instead of pre-visualizing the result, you fire away at varying exposures, hoping you catch the "right" exposure in one of the frames. Ansel Adams invented the Zone System, as a way to produce a print that looks the way the photographer wants the image to appear, not necessarily how the subject actually appeared. A daylight woods scene can be photographed to appear as it really is, or light and washed out, or dark and forbidding. It's up to the photographer and how she or he exposes the film.

• In the real world, you can trust your exposure meter most of the time. The trick is to recognize situations where it will be wrong. Then be prepared to override its suggestion.