When a lens is focused on an object at a certain distance, other objects, both closer and farther than the focus distance, form larger circles of confusion. When the film is placed at a point corresponding to the lens focus distance, a clear image is produced. When the film is nearer or farther away from the lens than the corresponding lens focus distance, the image becomes blurred because of the larger circles of confusion caused by the intersection of light rays either in front of, or behind, the film plane.
Another factor affecting the circle of confusion is lens aperture. Decreasing a lens opening narrows the light rays passed by the lens. The narrower these rays, the smaller the circles of confusion when the image is not in perfect focus. In practice, this means that a small lens opening is used to record, as clearly as possible, several objects at varying distances. Even when the rays from some objects do not intersect perfectly at the film plane, the circles of confusion ahead or behind the film are negligible and still appear as a sharp image.
The size of the permissible circle of confusion depends on the film format size and the manner in which the film will be used. Experience has shown that the permissible circle of confusion should not exceed about 1/1000 of the focal length of the lens.
The minimum circle of confusion of most lenses is sharp. Consequently, the distance that the focal plane small. Thus the focal plane can be moved slightly and can be moved forward or backward from the plane of yet retain an acceptable sharp image. However, as the sharp focus and continue to produce an image of distance of the movement is increased, the circle of acceptable sharpness is termed the depth of focus. This confusion becomes greater and the image becomes less depth is always within the camera.