Filming & Production
Vignetting
Also called: vignette, corner darkening
Optically it comes from the glass. Wide apertures and wide focal lengths both drop light at the edges, and a filter ring or a badly fitted hood can physically clip the corners, which is mechanical vignetting. Most editors carry lens profiles that remove it in a click. The other sort is added on purpose to pull a viewer's eye towards the middle of the frame, and it works only while it stays subtle enough that nobody consciously notices. A heavy vignette is one of the quickest tells of an amateur edit, especially when the lens was already vignetting and the effect lands on top.