Bruce Barnbaum - The Art of Photography - An Approach to Personal Expression

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The other confusion comes from the fact that the f/stop number is actually a ratio of the lens opening to the focal length of the lens, so we are really dealing with an inverse amount, or ¼, , , , , , , , etc. This is how we end up with the rule, “The bigger the number, the smaller the hole,” i.e., the larger the f/stop number, the smaller the aperture.

The two methods of doubling or halving the exposure, via aperture or shutter speed, point out a very important relationship: one zone is equal to one stop . When you “open up a stop—for example from f/11 to f/8—you are at the same time giving one zone more exposure. In the same way, when you change the shutter speed from, say, second to second, you are also “opening up one stop” or giving one stop greater exposure.

Now that you know what the zones are and how you can jump from one to another with your camera, set that information aside for a while as we look at the workings of the light meter. By combining the knowledge of the zones with an understanding of the light meter, the zone system becomes immediately clear.

Note

As a brief technical aside, it is worth explaining aperture numbers, which seem so arbitrary and senseless at first. The formula for the area of a circle is A=πr 2, where A is the area of the circle, r is its radius, and π=3.14159. If you square each of the aperture numbers (i.e., multiply it by itself) and plug them into the formula as r 2, they form a simple geometric progression: 4, 8, 16, 32, 64, 128, 256, 512, etc., so that each opening is half or double the previous one.

The Light Meter—How it Works

In order to fully understand the light meter, let’s refer to it by a different name for awhile. The name “light meter” is a bit confusing. We’ll call it a “gray meter” because its only function is to tell the proper exposure for medium gray, or Zone 5. All gray meters do the same thing: they average light within the viewing angle of the meter and give you a reading of the proper exposure for Zone 5. That is the only thing they do!

Note

All gray meters do the same thing: they average light within the viewing angle of the meter and give you a reading of the proper exposure for Zone 5. That is the only thing they do!

The gray meter has no idea what it’s pointed towards. It could be a dark coat in a hallway, a field of snow under bright sun, a person’s face under overcast skies, or anything else you can think of. All it knows is the amount of light that impinges on its lightsensitive cells. Since it has no idea what it’s looking at, it cannot give the proper exposure for the item, so it gives the Zone 5 exposure! For this reason, it should logically be called a gray meter.

Zone 5 may be just the right exposure for many things—perhaps a person’s face under overcast skies (though it seems a bit too dark to me)—but it surely is not the proper exposure for sunlit snow! The difference between you and the gray meter is that you can think! Recognizing that the meter always gives a Zone 5 reading, you can supply your knowledge of the tonal scale to determine the proper exposure.

Let’s take the case of sunlit snow. If the snow has modulations—small hills and valleys—and perhaps even some shadows, you may want it somewhere between Zone 7 (easily seen light gray textures) and Zone 8 (very light gray, almost white—no real textures). Even though snow is white, you probably want to avoid a blank white area lacking in detail. Suppose the gray meter reads f/16 @ second exposure. Of course that exposure would yield medium gray (Zone 5) snow! So, how do you get from the metered reading to the desired reading?

Start with the metered reading and first double the exposure by opening up one stop, which is the equivalent of increasing the exposure by one zone. From f/16 @ second, you can go to f/16 @ , which doubles the exposure and places the snow in Zone 6. (Remember that second is twice as long as second.) Alternatively, you could change the aperture instead of the shutter speed and achieve Zone 6 with a setting of f/11 @ second.

Next, double the exposure again to place the snow in Zone 7. From f/16 @ second, you can go to f/16 @ second, again using the shutter speed to double the exposure. (By changing the aperture you can achieve the same thing, going from f/16 @ second to f/11 @ second.) If you want to go to Zone 8, you can again change either the aperture or shutter speed to move up another zone on the scale.

Each doubling of the exposure makes the snow one zone lighter in tone. You can even go a half tone lighter by using the aperture setting between the f/stops shown on the camera. Thus, to place the snow in Zone 7½, go from the Zone 7 setting of f/16 @ second to a point halfway between f/16 and f/11 @ second. With this exposure setting, the snow will come out at Zone 7½, and it will show very light textures without being blank white.

With this example, you have learned how to expose negatives using the zone system. The critical point to remember is that the gray meter always gives a Zone 5 reading . You must open the aperture or increase the shutter speed if you want lighter tones than Zone 5. You must close down the aperture or decrease the shutter speed if you want tones darker than Zone 5. Then, by maintaining a mental image of the tonal value of each zone, you can determine exactly how much to open up or close down to obtain your desired exposure.

It may be confusing at first to realize that when you are standing in front of a bright object (such as sunlit snow), you must open up the lens to make it bright. It seems logically correct to close down the aperture from your meter reading because the object is so bright, but the opposite is true. You always start from the Zone 5, medium gray meter reading, and if you want your subject to have tones lighter than Zone 5 you must give it additional exposure above the meter reading to make it light . Similarly, starting with the Zone 5 medium gray meter reading, if you want your subject to have darker tones than Zone 5, you must give it less exposure than the meter reading to make it dark . Just as you must interpret the scene rather than merely “shoot” it to make your point of view visible, you must also control the negative densities. You can do so if you recognize that your starting point is always medium gray, the exposure that the gray meter always gives you!