numerical aperture (na)

A value used to express the brightness or resolution of a lens optical system. The numerical aperture, usually indicated as NA, is a numerical value calculated from the formula nsinØ, where 2Ø is the angle (angular aperture) at which an object point on the optical axis enters the entrance pupil and n is the index of reflection of the medium in which the object exists. Although not often used with photographic lenses, the NA value is commonly imprinted on the objective lenses of microscopes, where it is used more as an indication of resolution than of brightness. A useful relationship to know is that the NA value is equal to half the inverse of the F number. For example, F 1.0 = NA 0.5, F 1.4 = NA 0.357, F2 = NA 0.25, and so on.