When the point you are attempting to focus is a point light, the shape of the point light on the film is exactly where the lens has refracted each ray from the point light to the lens. In composite, this will result in a blur kernel in exactly the shape of the lens.
When the point you are attempting to focus is a point light, the shape of the point light on the film is exactly where the lens has refracted each ray from the point light to the lens. In composite, this will result in a blur kernel in exactly the shape of the lens.