Paul Jolly Photography Chapter 24.2

Angle vignetting

This is common to almost all lenses but software built in most cameras can compensate for this effect. This effect is so similar in result to Physical vignetting we use the same generic name; when you talk about the effect just be sure you know which version is being discussed.
When light enters the lens on its axis there is a minimal amount of glass the light has to travel through. A ray of light entering from any other angle has to travel through more of the glass; the more glass a ray travels through the more the glass absorbs, attenuating the required ray and making the final image darker. This effect is most pronounced on wide angle lenses. (A camera lens is a complex structure, do not infer glass thickness from the simple diagrams on these pages).
Photographers using RAW output from their cameras will already know of this problem and compensate for it in their workflow.
This effect can be diminished somewhat by using a smaller aperture (larger f number) because this restricts the usable light towards the centre portion of the lens reducing the edge attenuation.
Although Angle and Physical vignetting work in opposite directions they do not cancel out and can produce some strange effects if you are not careful.