Optical Vignetting
Corner darkening caused by the lens barrel physically clipping the light cone reaching off-axis parts of the frame.
Measured in
stops (EV)
In depth
Because it is caused by the barrel obstructing oblique rays, optical vignetting is worst wide open and clears up substantially by two stops down. It is also what shapes cat's-eye bokeh at the frame edges. A modest amount is often left in deliberately, since it draws the eye toward the centre of the picture.
Examples in practice
An 85 mm f/1.4 loses about two stops in the extreme corners wide open, recovering to under half a stop by f/2.8.