Image circle.
Although image areas on most cameras are rectangular or square, lenses do not cast rectangular or square images onto the focal plane. Instead they cast circular images. This effect can be seen particularly with extremely wide angle fisheye lenses (8mm lenses for 35mm cameras) - the image is a circle blurring to black around the edges.
Lenses are designed, therefore, to project an image circle larger than the image area used by the camera.This means that there are four curved areas at each edge of the rectangle which are essentially cropped out in the final image. Also, because there’s a lot of light falloff towards the edge of the image circle the desirable area is actually a bit smaller than that to avoid vignetting.
Normally this doesn’t really affect a photographer - lenses are designed to project image circles larger than the image area and that’s that. However, there are two circumstances under which it becomes an issue.
First, it explains why you can’t really attach lenses designed for 35mm cameras to medium or large format cameras even if you were to find a physical lens to camera adapter. The image circle cast by a lens for a 35mm camera is simply too small to cover adequately the image area of a medium or large format camera - you’d end up with a circular image surrounded by black.
Second, the size of the image circle determines the coverage range of a camera which supports movements. If you move the lens too far in relation to the image area you’ll start getting vignetting and then dark areas appearing in the final image.
cf. cropping, falloff, fisheye lens, image area, movements.
Entry last updated 2002-04-03. Term 631 of 1487.
Previous term: image area.
Next term: image editor.