Vignetting

Optical vignetting

Most photographic lenses exhibit optical vignetting to some degree. The effect is strongest when the lens is used wide open and will disappear when the lens is stopped down by a few stops. A photograph or drawing whose edges gradually fade into the surrounding paper is called a vignette.

Vignetting is stronger at wider apertures of the lens (i.e. f/4.8), so when shooting photographs  you should use a small aperture (i.e. f/8 or f/11). and this will give better optical quality, too.