Before revision: Instead, the angular aperture of a lens (or an imaging mirror) is expressed by the f-number, written N, where N is the f-number given by the ratio of the focal length to the diameter of the entrance pupil : This ratio is related to the image-space numerical aperture when the lens is focused at infinity.
After revision: Instead, the angular aperture of a lens (or an imaging mirror) is expressed by the f-number, written , where is the f-number given by the ratio of the focal length to the diameter of the entrance pupil : This ratio is related to the image-space numerical aperture when the lens is focused at infinity.