Wide angle lenses for large- and medium-format cameras
often vignette the image—the film is not uniformly
exposed, but instead illumination falls off away from the
optical axis of the lens, underexposing the edges and
corners. Traditionally, optical “centre filters”
have been used to compensate for vignetting, but they require
additional exposure time which may be impractical. Pnmctrfilt
is an addition to the Netpbm image processing toolkit which
simulates the effect of a centre filter in an image exposed
without one. Options allow emulating a wide variety of optical
filters.