Exposure compensation

One of the most important functions on your camera is the exposure compensation feature. When the built-in meter doesn’t provide the perfect exposure, exposure compensation allows you to adjust the image lighter and darker in 1/3 f/stop increments.

Many beginning photographers assume incorrectly that you can alter exposure by adjusting the lens aperture and shutter speed. This is true only when the camera is set to manual exposure mode. When the camera is set to an automatic exposure mode -- Program, Aperture priority, Shutter priority -- changing the lens aperture or shutter speed does not change the exposure. The reason is simple. The lens aperture and

shutter speed are linked together when given a specific ISO.

For example, the photograph on the left side of the previous page was exposed at 1/15, f/16, and 250 ISO. If I were on Aperture priority (because I wanted to choose a specific aperture for extensive depth of field) and I wanted to decrease the exposure to make the picture darker, what happens if I closed the lens down one f/stop from f/16 to f/22? Would the picture be darker?

No, it wouldn’t.

This is because the automatic exposure mode links my choice of lens aperture, f/16, to the shutter speed that produces the correct exposure. In this example, the shutter was 1/15. If I changed the aperture to f/22, believing this would decrease the light reaching the digital sensor, that would be erroneous thinking. The shutter, in response to the reduction of light by one stop, would automatically change to 1/8th of a second to compensate for the loss in light.

In other words, one f/stop reduction in light by the aperture and one f/stop increase in light by the shutter produces the exact same exposure. Look at the numbers:

1/15 and f/16 = 1/8 and f/22

Therefore, adjusting the lens aperture when using Aperture priority doesn’t affect the exposure. It just changes the depth of field. In the shot below of a fer-de-lance snake from Costa Rica, a photographer might want to lighten the image to bring out more detail in the mosses and in the dark markings on the venomous snake. Opening the lens aperture when shooting with Aperture priority exposure mode is not the way to do it.

The same holds true for using Shutter priority. That’s what I used in the picture below and at right. If you want to adjust the exposure by changing the shutter, the lens aperture will automatically change to produce the same exposure.

The only way to make a picture lighter or darker in-camera when using Av, Tv, or Program mode is to use the exposure compensation feature. Or, if you are shooting on manual mode, you can adjust the exposure by changing the shutter speed and/or lens aperture. On manual mode, the exposure compensation feature is disabled.

With mirrorless cameras, one of their great advantages is you can see the exposure changes in the image through the viewfinder. This takes all the guesswork out of exposure. §