Image circle
The image circle is the cross section of the cone of light transmitted by a lens or series of lenses onto the image plane. When this light strikes a perpendicular target such as photographic film or a digital camera sensor, it forms a circle of light – the image circle. Various sensor aspect ratios may be used which all fit inside the same image circle, 3:2, 4:3, 16:9, etc.
A lens to be used on a camera that provides movements must have an image circle larger than the size of the image format. To avoid vignetting, a photographer using a view camera must ensure that the area remains within the image circle ; a tilt/shift lens or perspective-control lens used on a small- or medium-format camera usually has mechanical limitations that keep the frame area within the image circle.
