Adobe ® DNG Flat Field is a plug-in for Lightroom to correct an optical
phenomenon called shading (also known as "lens cast"). Shading is a more
general form of standard lens vignetting: with shading, the light falloff can be
asymmetric and can have a color cast (e.g., green on the left side of the image,
and magenta on the right side). It is most often seen with mirrorless camera
systems. Examples include medium format digital backs on technical cameras,
and Micro Four-Thirds or Sony NEX cameras with third-party, non-retrofocus
lenses.
The easiest way to fix shading is with a calibration image. When you take a
regular photo, you also shoot a reference or calibration shot (e.g., through a
plastic diffuser card) with the same optical setup. For best results, use the same
optical configuration (i.e. same lens, f-stop number, focal length, focal distance,
etc.) when shooting your regular photo and the calibration shot. Software like the
DNG Flat Field plug-in can then analyze the calibration shot, and remove the light
falloff and color cast from the corresponding real photo.