Vignette

The corners fall darker than the centre, so the eye is held where the picture is brightest. It is the quietest way to compose a frame after it is shot.

It also carries age. Older and faster glass vignettes more, so the darkening reads as an older camera even when nothing else in the shot does.

Natural falloff is optical. Light reaching the corner of the frame arrives at an angle and travels further, and the result follows a steep curve, so the corners darken well before the sides do.

Mechanical vignetting is something physically in the way: a hood built for a longer lens, a stack of filters, a matte box set too close. That kind has a harder edge and can clip the corners outright.

Wide lenses and wide apertures show more of it. Stopping down reduces natural falloff, which is why a vignette asked for at f/16 has to be added rather than captured.

Use it to hold attention in the centre of a busy frame, to age footage, and to seat a subject that would otherwise float on an even background.

Skip it when the corners carry information. A vignette buries whatever is out there, which is a problem in a wide shot that the audience is meant to read across.

Cameo lighting produces a lit subject against a black surround by controlling the light itself. The background goes dark across the whole frame, including the middle of each edge.

The test is the sides of the frame. A vignette leaves the middle of each edge brighter than the corners, because the falloff is radial. Cameo lighting has no such pattern, since the darkness comes from an absence of light rather than from the lens.

The result comes back with a hard circular edge, which is a mask. Keep the falloff gradual, and say it fades in over a wide band rather than stopping at a line.

The other failure is a vignette so heavy the corners go to pure black. Give it a strength, such as one stop down at the corners, so the model has a target.