where the numerical aperture "A" is equal to the aperture's radius "d"/2 divided by R', the distance from the center of the Airy pattern to the "edge" of the aperture. Viewing the aperture of radius "d"/2 and lens as a camera (see diagram above) projecting an image onto a focal plane at distance "f", the numerical aperture "A" is related to the commonly-cited f-number "N= f/d" (ratio of the focal length to the lens diameter) according to formula_28; for NÂ»1 it is simply approximated as formula_29. This shows that the best possible image resolution of a camera is limited by the numerical aperture (and thus f-number) of its lens due to diffraction.