Flat-field correction

Not to be confused with Field flattener lens, which improves focus uniformity.
This picture is darker at the edges. This variation is called vignetting, and can be corrected by selectively brightening the perimeter of the image.

Flat-field correction (FFC) is a digital imaging technique to mitigate the image detector pixel-to-pixel sensitivity and distortions in the optical path. It is a standard calibration procedure in everything from personal digital cameras to large telescopes.