Vignetting in Camera Lens
When a wide angle lens of very broad
coverage is used, or when a view camera lens is used with a film format
larger than it was designed to cover, there may be vignetting, in which
the illumination of the film drops off toward the corners of the image.
The corner light intensity may be as much as one-half stop, or
occasionally a full stop, less than at the center of the image. It can
be at least partially corrected by closing the lens aperture well down.
Actual image vignetting should not be
confused with the darkening that occurs at the corners of a camera
viewfinder that incorporates a ground glass screen (as in all view
cameras and many single-lens reflexes). This is a purely visual effect,
and is caused by differential scattering of light rays approaching the
viewing screen at varying angles; the center rays approach straight on,
and are scattered with full effect, while the rays at the image corners
approach the screen at an angle and some of the light is reflected off,
away from the eye. If you move your eye toward the corner, you can see
the image brighten. This has no effect on film exposure; but this
viewfinder effect makes it impossible to judge the amount of true
vignetting by examining the image in the viewfinder.
Vignetting will become obvious only in
the appearance of the developed negative or transparency. With lenses
that display the effect, you should test to see how much aperture
closing will be required for adequate correction, so that you will know
what to do in the future.
If some non-correctable vignetting is
present, and you are using black-and-white negative film, it is a good
idea to overexpose the film a little to be sure that the corners of the
film receive adequate light.
Then you can correct the center
overexposure during printing. Any underexposure rapidly reduces image
contrast, which cannot be corrected in printing; but moderate
overexposure has very little effect on image contrast, and so is easily
compensated. Vignetting in color slides cannot be corrected in any