Exposure Compensation and Exposure Value
An exposure compensation changes the exposure that the camera has determined to make your picture brighter or darker.
The most common usage is certainly to avoid over- and underexposure.
An understanding whey this is still necessary despite all modern technology in the cameras is very helpful and described in the primer about exposure metering.
On the left you can see an example for an overexposed part of an image and the same part with an exposure compensation:
Exposure compensations are set on digital cameras with values like e.g. +1.0 or -1.0, additional values inbetween are possible in steps of ⅓ or ½.
Here are two samples of camera displays while setting an exposure compensatioin. Yours will look similar with a display of the compensation value or a scale:
Exposure compensations with a positive values make your pictures brighter, negative ones darker. My rule of thumb: Usually +/-1 is enough and you rarely need more than +/-2. The difference between the first two images above is a compensation of -1.0, between the two photos of an evening sky -0.5.
Here is a series that illustrates how the brightness changes. It is taken with a Nikon D7000, other cameras can react to the exposure compensations a bit differently.
The number values represent changes measured in exposure values, sometimes indicated by a display of „EV“ on the camera
A compensation by one exposure means to double or halve the amount of light for exposure.
Accordingly the amount of light will change by a factor of 2 or 1/2 for each additional value:
|exposure compensation||means a change in the amount of light by a factor of…|
Intermediate values can be calculated with exponentiation but do not matter for understanding the principle.
The term exposure value and the logic behind that the quantity of light is changed not in a linear way but always by factor of 2 is important for understanding exposure because the parameters exposure time, aperture and ISO sensitivity follow the same principle.
Setting an exposure compensation is only an indication how much brighter or darker a picture should get.
It does not say if the camera should adjust the exposure time, the aperture or the ISO sensitivity to achieve it.
Background knowledge when you should set an exposure compensation is in the primer about exposure metering.
But you can also be pragmatic by just taking a picture, checking the exposure and if you do not like it, take another one with exposure compensation until you get it right.