Image circle
From The Glossary of Digital Photography
Each lens projects an image in the shape of a circle, not a rectangle, the shape seen in the viewfinder. The image circle needs to be large enough to cover the image sensor of a digital camera. Special digital lenses are now being manufactured that have a smaller image circle, or area of coverage, to fit the smaller image sensors often used. Thee manufacturers do this so that more-compact and less-expensive lenses can be created. It is important to note that these lenses may attach to other cameras (including film cameras), but that their image circle is not large enough to cover the larger sensor, or film area, and will vignette around the edges.
