How Digital Photography Works

Exposure equivalences are the basis of a common feature among digital—and film—cameras: exposure modes. The photographer

chooses among modes using a knob located conveniently near the shutter button. Typically the modes include

• Automatic Exposure—The processor works unassisted except for the light meter readings and information from the lens that
  suggest a shot might be a landscape or one that requires a fast shutter to defeat shaky hands. The chip’s software decides on both
  the shutter speed and aperture, usually choosing a combination that works perfectly fine for the average shot.


• Programmed Exposure—Similar to automatic mode, except that the photographer can change the shutter or aperture on-the-
  fly and that the processor responds by adjusting the aperture or shutter, respectively, to maintain the same exposure equivalence.


• Shutter Priority or Time Value—The photographer chooses the shutter speed, and the processor picks a complementary
  aperture setting. Shutter priority is useful when it’s important, for example, to have a fast shutter to freeze an action shot.


• Aperture Priority or Aperture Value—Just the reverse of shutter priority. The photographer chooses the aperture, and the
  processor picks the shutter speed. It’s useful for a portrait when the photographer wants to maintain a large aperture so the back-
  ground is blurred and unobtrusive.
  
  
  • Manual—For those times when the photographer has a better understanding of the lighting than the light
    meter does.
  
  
  
  

2

The photographer’s choice of exposure mode influences which exposure

setting, from among several EVs, the processor picks. The chart here

shows a multiprogram autoexposure system that allows the photographer

to select fully automatic, high-speed, or maximum depth-of-field without

having to pick any specific shutter or aperture setting. The exposure sys-

tem sticks to one of a few specific paths that run through the range of

aperture and shutter settings. The setting in each of the paths meets the

requirements of the photographer’s selection.

3

For the average photograph, for which the photographer has not cho-

sen any special priority, the exposure system chooses a setting among

the EVs shown here in yellow. For depth-of-field priority, the camera’s

processor chooses among the blue EVs. For action shots, the choices

are concentrated along the high–shutter speed EVs, marked in green.

CHAPTER 5 HOW DIGITAL EXPOSURE SIFTS, MEASURES, AND SLICES LIGHT 75