Image Circle; Circle of Illumination; Circle of Good Definition
The image circle, or circle of illumination is a circle of light on the focal or film plane. It is projected as a cone of light focused from the lens. In a digital camera the focal plane is at the surface of the digital image sensor.
The projected image circle has poor focus definition on the perimeter. Within the circle formed by the area of poor definition is another circle called the circle of good definition. This is the circle of acceptably sharp definition that forms the focused image. The effective area of the image sensor that records the image must lie within the circle of good definition.
In the case of a digital camera the circle of illumination is projected onto the focal plane at the surface of the image sensor.