||This diagram says that of of
light which comes to the film in the center, at the
utmost corners of the
negative (21mm away from center) less than 25% arrives
at the fastest aperture
of the lens. That means the corners are probably
underdeveloped by 2 f-/stops. Bad thing if you have
slide film used.
Closing down the lens 3 stops reduces the light loss to one stop (upper curve)
This diagram is from a newly, excellent (Retrofocus type) designed 21mm lens. This form of, in everyday speech "vignetting" is unevitable, and occurs even in the best designed wide angle lenses. If it would be vignetting it could be avoid in one or another way, probably by the lens designer.
But the only thing one can do against it belongs to the photographer: Either avoid very wideangle lenses, stop down at least two stops, change background, or use tolerant film (i.e. B&W negative)
||Left: Image of a brick wall laid by an
beginner. Original image (no distortion) above and heavily
barrel-distorted image (beneath)
Middle: perspective deformed but distortion-free picture (falling lines).
If you enlarge this picture large enough and look at it from close distance, the falling lines dissapear! This is what "natural perspective" means (don't do it with your monitor, please)