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David Busch’s Canon® EOS M Guide to Digital Photography by Lee Sheldon

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How the EOS M Calculates Exposure

Your Canon EOS M calculates exposure by measuring the light that passes through the lens and arrives at the sensor, using a pattern you can select (more on that later) and 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, which does represent middle gray; however, 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 ...

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