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The image circle is the cross section of the cone of light transmitted by a lens or series of lenses. When this light strikes a perpendicular target such as 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 (Adams 1980, 54). To avoid vignetting, a photographer using a view camera must ensure that the area remains within the image circle (Adams 1980, 56–57; 151–52; 157–61); a tilt/shift lens or perspective-control lens used on a small- or medium-format camera usually has mechanical limitations that keep the image area within the image circle.