Light fall-off (vignetting)

Especially wide-open, most lenses are brighter in the center than the edges. This is especially pronounced with wide-angles and some long teles. This phenomenon is known as light fall-off or vignetting.

A fairly mild case of vignetting. If a lens does no worse than this wide-open, it's pretty good.

Assessing vignetting: Take a test shot of a scene with even lighting. If there's significant vignetting, the center will be visibly brighter than the corners.

Excellent: No obvious vignetting at any aperture.

Good: Some vignetting wide-open; gone one or two stops down.

Not-so-good: Wide-open vignetting is bad enough to need post-processing. Some visible even stopped-down.

Bad: Vignetting bad enough to need correction in-camera (with a circular-graded neutral-density filter), or use of B/W negative film. You might see this on an ultra-ultra-wide rectilinear SLR lens, such as a 14 or 15 mm. It does happen on the widest rangefinder lenses currently available (the 12 mm Voigtländer Heliar, for example).

Ramifications: In my opinion, vignetting is close to a non-issue. It's extremely unlikely to be bad enough that it can't be pretty easily corrected in post-processing (with a curves tweak and circular-graded mask).

Remedies: Vignetting is easy to fix in post, unless it's really bad (over a stop and a half or so):

1. New Adjustmen Layer: Curves, blend mode Luminosity.
2. Pull up the curve so that the corners brighten up approximately to the original levels of the center. Don't worry about getting it exactly right: overshooting is OK.
3. Select mask channel. Put a circular, graded fill from black to white, centered in the frame.
4. Adjust translucency until vignetting mostly fades out. If you get a circular banding effect, redo the mask channel fill.