Vignetting

When you start using lenses and lens accessories in ways the manufacturer never intended, you sometimes get surprised.  In close-up photography, one form of surprise is vignetting.

Vignetting means darkening of the corners of an image relative to its center. It happens to some extent with almost every lens, with wide-angle lenses being the worst offenders.  If the fall-off occurs gradually, the viewer may not notice it, and it can even add to the drama of an image, since the lighter center tends to draw the eye inward.  In close-up photography with various accessories stacked onto a prime lens, abrupt vignetting can occur, where the light falls off almost to zero very quickly.   This makes the image unusable, unless you're willing to crop out the vignetted part.

One common way to get vignetting in close-up photography is to invert a normal photographic lens and attach it to your prime lens so that the front elements are facing each other.  You can buy special rings, called macro rings, that screw into the filter threads of each lens to hold them together.  We underwater photographers don't have to worry much about this kind of vignetting, since the reversed-lens scheme usually results in subject distances too short to be useful underwater.

A less-common way to get vignetting is to have incompatibilities between the prime lens and a tele-converter.  If the lens and the tele-converter are manufactured by the same company, they'll usually tell you whether the combination will work without vignetting.   However, in our search for magnification, some of us add extension tubes between the lens and the tele-converter.  This can sometimes keep a combination that would otherwise vignette from vignetting, or it can cause the problem in an otherwise compatible lens and converter.

One otherwise-attractive combination that suffers from minor vignetting is the 105 mm f/2.8 AF Micro-Nikkor, TK-11 and TK-12 extension tubes, and the Nikon TC-14A tele-converter.  The vignetting occurs only at the extreme corners, and would probably be covered if the film is placed in a slide mount, but it's there and it's a problem for those of us who like to be able to print the whole frame.  This combination only vignettes when the lens is focused near the maximum magnification (a little over 2:1). The (hard to find) Nikon TC-14 converter does not have this problem.