Vignetting | Both an aberration and a creative effect

Vignetting (also called corner shading) is where the lens doesn't produce the same brightness at the edges of the frame as it does in the middle. It can also happen when the lens’s image circle is just a little too small for the sensor/film area and the areas furthest from the centre (the corners) appear darker. It’s common with cheaper lenses or zooms at the limits of their performance – wide open and minimum focal length, for example.

You can see it in this shot, where the top corners are clearly darker. It's possible to fix this using software (Photoshop's Lens Correction filter, or DxO Optics Pro, for example), but it's better if the lens doesn't produce vignetting in the first place.

Vignetting can also be applied deliberately using software to produce an old-fashioned effect, though photographers prefer to add it by choice and under careful control as and when they want it.