The Meters Dilemma

Unfortunately, finding the correct exposure isn't always that simple. The problem is that light meters have no way of actually "seeing" the objects they measure. An exposure meter is simply a device that measures quantities of light and the problem is that both a light surface and a dark surface will reflect the same amount of light if the light falling on them is changed. On a bright, sunny day a dark wall might read meter number 11, but so might a light wall on a cloudy day! To your eye, the light and dark walls are obviously very different, but to the meter, they are both meter number 11.

Think about this for a minute. "Seeing" is a complex process of visual perception that is much more than simply measuring amounts of light. For example, a piece of white paper looks white to you in both a bright light and a dimly lit room.

The light meter is responding to the change in the incident light, but you can judge that the dark wall should print as Zone III and the light wall as Zone VII.

Unfortunately, the meter still has the responsibility of suggesting the correct exposure for you to use. In this example, given that the meter reads both walls as number 11, it obviously cannot suggest the different exposures that the two walls would require. Light meters are designed to get around this problem in a simple way: They lie! Because the meter can't tell that the dark

Exercise: How Light Meters Really Work 39

wall is dark or that the light wall is light, it will simply pretend that both walls are middle gray. In this way, the exposure that it recommends can at worst only be half-wrong. Of course, if the wall had been gray in the first place, the meter's recommended exposure would have been perfect.

The fact is that all light meters are programmed to give you an average gray exposure for whatever amount of light they are measuring.*

In the language of the Zone System, we would say that all light meters will automatically place any light reading on Zone V. This is very important for you to understand and remember.

This principle explains why the exposures I used in the previous chapter changed the way they did. Remember that I said that the meter reading for the white wall was 1/60 @ f/22 and for the black wall it was 1/60 @ f/5.6. In both cases the meter is suggesting an exposure that will make each wall print as middle gray. To make a black wall gray the meter has to recommend an exposure that makes it lighter. (Gray is lighter than black.) Conversely, to make a white wall gray it needs to recommend an exposure that makes it darker. (Middle gray is darker than white.)

A simple demonstration that you can do with your own camera and film or with Polaroid materials will illustrate how this works.

Note: I strongly recommend that any reader approaching the Zone System for the first time should stop and work this demonstration through to the end before going on. Not only will it graphically illustrate the way light meters operate, but it will also function as an ideal warm-up for Zone System testing.

Before you begin, it is a good idea to have your light meter and shutter speeds tested for accuracy and consistency. If your technician advises you that your equipment tests within reasonable limits, proceed with this demonstration. If not, leave the equipment with the technician to be adjusted.

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