Nikon manual How the D7000 Calculates Exposure, Correctly Exposed, Stops Versus Stops

Chapter 4 ■ Getting the Right Exposure 101

F/STOPS VERSUS STOPS

In photography parlance, f/stop always means the aperture or lens opening. However, for lack of a current commonly used word for one exposure increment, the term stop is often used. (In the past, EV served this purpose, but Exposure Value and its abbreviation have since been inextricably intertwined with its use in describing Exposure Compensation.)

In this book, when I say “stop” by itself (no f/), I mean one whole unit of exposure, and am not necessarily referring to an actual f/stop or lens aperture. So, adjusting the expo- sure by “one stop” can mean both changing to the next shutter speed increment (say, from 1/125th second to 1/250th second) or the next aperture (such as f/4 to f/5.6). Similarly, 1/3 stop or 1/2 stop increments can mean either shutter speed or aperture changes, depending on the context. Be forewarned.

How the D7000 Calculates Exposure

Your D7000 calculates exposure by measuring the light that passes through the lens and is bounced up by the mirror to sensors located near the focusing surface, using a pat- tern 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; 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 exposure based on the white cat will cause the gray cat to appear to be black, while an exposure based only on the gray cat will make the white cat washed out.

This is more easily understood if you look at some photos of subjects that are dark (they reflect little light), those that have predominantly middle tones, and subjects that are highly reflective. The next few figures show some images of actual cats (actually, the same cat rendered in black, gray, and white varieties through the magic of Photoshop), with each of the three strips exposed using a different cat for reference.

Correctly Exposed

The three pictures shown in Figure 4.4 represent how the black, gray, and white cats would appear if the exposure were calculated by measuring the light reflecting from the middle, gray cat, which, for the sake of illustration, we’ll assume reflects approximately 12 to 18 percent of the light that strikes it. The exposure meter sees an object that it