Controlling Exposure

Automatic exposure control is one of the most useful features of  

a digital camera. Itís great to have the camera automatically deal  with the exposure while you concentrate on the image. This is espe-cially helpful when photographing action scenes where there isnít 

time to evaluate the situation and then set the controls manually.

You shouldnít, however, always leave the exposure to the automatic system. 

At times the lighting can fool any automatic exposure system into producing  an underexposed (too dark) or overexposed (too light) image. Although you  can make adjustments to a poorly exposed image in a photo-editing program,  youíve almost certainly lost image information in the shadows or highlights  that can’t be recovered. You will find it better in some situations to override  the automatic exposure system at the time you take the picture.

Situations in which you might want to override automatic exposure often  involve scenes with interesting or unusual lighting. For example, you need  to take control when you photograph into the sun, record a colorful sunset,  show the brilliance of a snow-covered landscape, or convey the dark moodi-ness of a forest. In this chapter youíll learn how to use your camera controls  to get just the exposure you want.

Photography begins with an exposure when you press the cameraís shutter  button. The shutter opens and light reflected from the scene enters through  the lens and strikes the image sensor to create the image. By controlling  how much light reaches the sensor you control how light or dark the picture  isóone of the most important aspects of photography. When a scene has both  very light and very dark areas, getting the perfect exposure is a lot like park-ing a large car in a small garageóthere isnít a great deal of room for error.  Your goal is to retain details in both the darkest and lightest areas so pure  white is used only for spectral highlights such as reflections, and pure black is  used only for those few areas of the scene that are black with no details.