Irrespective of make, model and specifications (focal length, f/number etc) all camera lenses produce a circular image on the sensor plane. This image circle is called the ‘Circle of Illumination’. Within the circle of illumination is a slightly smaller circle known as the ‘Circle of Good Definition’. A sharp image could be formed by the camera sensor only from within the circle of good definition and hence it is also known as the covering power of the lens. One thing to understand is that the covering power of lens increases when one focuses on objects closer to the camera and also when using narrower apertures.
|lens coverage circle of illumination circle of good definition|
If you look closely at the lens coverage illustration given above the red outer circle represents the circle of illumination. The smaller green circle within the red one represents the circle of good definition. An image formed using light from any point in the circle of good definition will be sharp. The orange extension to the otherwise centered image area represents shift movements; when excessive shift is applied the image utilizes light from areas between the circle of good definition and the circle of illumination which causes vignetting and deterioration of image quality. This is the reason why circle of illumination and also circle of good definition formed by tilt shift lenses are much larger than that of ordinary lenses.
For regular lenses the covering power will only be a little wider than the sensor but for special lenses like the tilt shift lenses covering power has to be much wider to take account of any shift and tilt movements that is possible with these type of lenses. It is critical that the sensor area remain within the covering power of the lens even with the maximum amount of tilt or shift possible or else the corners of the image will blur into a dark vignette as the sensor records light from the edge of the circle of illumination.