Cone of rays
When an object is imaged by a camera , a large number of beam cones are created . Every point of the subject to be recorded is projected through the lens onto the film plane . The (usually round) surface of the objective lens creates a mostly oblique cone, the tip of which falls exactly on the plane of the film, this point is precisely focused .
In contrast, if an object point is unsharp, the apex of the cone lies in front of the film plane or behind it. In both cases a truncated cone point is projected into the film plane. This small area of intersection is the so-called blurring circle or circle of confusion , which becomes a slightly flattened ellipse at points very far from the axis .