Nikon D6 Reference Manual page 1029

D
Distortion Control
•
Note that greater amounts of distortion control result in more of the
edges being cropped out.
•
Distortion control may heavily crop or distort the edges of copies
created from photographs taken with DX lenses at image areas other
than [DX (24×16)].
N The Retouch Menu: Creating Retouched Copies 977