Flat Field Correction
Dividing an image by a frame of evenly illuminated field to remove vignetting, dust shadows and photosite sensitivity variation.
Formula / rule
Corrected = (light − dark) / (flat − bias)
In depth
Where dark frames handle additive defects, flat fields handle multiplicative ones: anything that makes part of the frame receive or record proportionally less light. The flat must be shot through the same optical path at the same focus and aperture, since a change in either alters the vignetting and the dust positions.
Examples in practice
A flat field shot against a uniformly lit white screen removes both lens vignetting and every dust shadow in one operation.