How the Sony Alpha Calculates Exposure

Your A65 calculates exposure by measuring the light that passes through the lens to reach the sensor, based on the assumption that each area being measured reflects about the same amount of light as a neutral gray card that reflects a “middle” gray of about 12- to 18-percent reflectance. (The photographic “gray cards” you buy at a camera store have an 18-percent gray tone; your camera is calibrated to interpret a somewhat darker 12-percent gray; I’ll explain more about this later.) That “average” 12- to 18-percent gray assumption is necessary, because different subjects reflect different amounts of light. In a photo containing, say, a white cat and a dark gray cat, the white cat might reflect five times as much light as the gray cat. An ex....