Exposure Compensation and Auto Exposure Bracketing
A camera's automatic exposure meter is not always able to calculate the optimal exposure. As a result, exposure compensation is used to adjust the meter's exposure value (EV). For example, when an image contains strong reflections, the reflected light can cause the image to become dark, or underexposed. On the flip side, if you take a picture of that same person against a black background the camera will want to overexpose the image to get away from having clipped shadows. When this happens, applying plus compensation creates an optimal exposure.
Exposure compensation can increase (brighter) or decrease (darker) the standard exposure set by the camera. The change in light amount of 1 EV is equivalent to changing the shutter speed or aperture value one level.
If you are at 0 exposure compensation, you will see a mark in the middle of your exposure meter on your cameras LCD or in the viewfinder. If you spin the wheel in either direction, you will see the mark move either up or down.
Although you can set the exposure compensation up to 5 stops in 1/3 stop increments, the exposure compensation indicator in the viewfinder and on the LCD panel can only display the setting up to 3 stops. If the exposure compensation amount exceeds 3 stops, the end of the exposure level indicator will display < or > sign.
The exposure compensation amount will remain in effect even after you set the power switch to <OFF>. To cancel exposure compensation, set the exposure compensation amount back to center indicator.
In P, Tv, and Av modes, the camera sets either the shutter speed or aperture, or both, to produce a good exposure, and the meter displays the current Exposure Value setting. Because you don’t need the meter to tell you whether exposure is okay in these modes, the meter instead indicates whether you enabled Exposure Compensation, a feature that forces a brighter or darker exposure than the camera thinks is appropriate.
Assume you are shooting in aperture priority, and you have chosen your aperture based on the subject matter but find that your image is constantly dark. By adding +1/2 or +1 stops via the exposure compensation on your camera you will lighten the image overall. When in aperture priority, because you want the aperture to remain the same, the camera will use a slower shutter speed to compensate.
Conversely in shutter priority mode, by adding +1/2 or +1 stops via the exposure compensation on your camera, the camera will open the aperture, allowing more light to reach the sensor (or film). The reverse is true if you are finding your image is too light, dialing in a minus exposure compensation number will darken your image incrementally.
Another classic example is photographic birds or aircraft in flight. Even the best exposure meters will generally expose for the sky leaving the bird or plane dark in comparison. Again dialing some plus exposure will lighten up your subject although possibly at the expense of a washed out sky.
A typical example that may over expose is a forested landscape; the darkness of the trees can very often trip up a meter. Here, using minus compensation will help correct exposure.
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