To understand how Zone System theory applies to exposure, let’s first consider the different types of light meters 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 (see Figure 5.1). 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.