f/No. and the radiometry of image forming optical systems with non-circular aperture stops

摘要

The f/No. concept is extended to include image forming optical systems with non-circular aperture stops. The basic principles of classical radiometry, geometrical optics, and physical optics are described by a single simple formula that is used to derive f/No. definitions which account for effects from diffraction, aberrations, and arbitrary stop shapes. In general, the effective f/No. of an optical system is a strong function of stop shape if the stop is very narrow in one or more dimensions, with equivalent circular stop areas of ～ 0.5 ≤ f/No. ≤ 1. Practical methods for computing the effective f/No. are discussed.