To understand how Zone System theory applies to exposure, let's first consider the different types of light meters that are available and how they function.

Light meters come in two general types: meters that measure what is called incident light, and meters that measure reflected light.

Incident-light meters measure the light that falls on the subject from the source of light (Figure 24). Incident-light meters are especially useful in lighting studios or on movie sets where the contrast can be controlled with fill-lights or reflectors. Under those conditions, your primary concern is to make sure that the amount of incident light falling on your subject does not change from one shot to another.

Reflected-light meters measure the light that bounces off the subject to the camera and film (Figure 25).



FIGURE 25 Reflected-light reading.

FIGURE 25 Reflected-light reading.

As we discussed earlier, with the Zone System, we are primarily interested in measuring the contrast of a given scene. The question is: How much light is coming from one area as opposed to another? Only a meter that measures reflected light can effectively tell us this difference.

There are three different types of reflected-light meters: spot meters, wide-angle meters, and built-in camera meters.

The first two types of reflected meters differ in terms of what is called their angle of incidence. A wide-angle meter "sees" at an angle of approximately 30 degrees, while a spot meter reads at an angle of only 1 degree or less in some cases (Figure 26).


FIGURE 26 Wide-angle and spot meters.


FIGURE 26 Wide-angle and spot meters.

Light Measurement 37

This is similar to the difference between looking through a straw as opposed to looking through a cone. The advantage of using a spot meter is that it allows you to measure the light from small, isolated areas very accurately at a distance. Imagine that you need to know how much light is reflected by a model's face. With a wide-angle meter, you would have to get very close to avoid including her hair in the reading. This is less of a problem if you can approach the subject, but if you are photographing distant mountains or performers on a stage, a wide-angle meter can make getting an accurate reflected reading very difficult, if not impossible.

The acceptance angle of a built-in meter varies depending on the kind of meter the camera uses and the focal length of your lens.

Thus far, I have talked about "exposing for the shadows," or "making the background Zone III," without explaining how this is actually done.

The problem of how to calculate the proper exposure becomes easy when you understand more about what a light meter is actually designed to do.

Light meters are programmed to perform two different but related functions: Light Measurement and Exposure Recommendation.

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