Vignetting
From ScienceZero
Vignetting is a reduction in brightness away from the centre of the photograph. It can be caused by obstructions in the lens or outside the lens, by the optical design itself or by the way the sensor reacts to light entering at different angles.
Vignetting is easiest to see in the sky or other areas of uniform brightness and colour. If vignetting is desirable or not depends on the artistic vision of the photographer.
