Lens resolving power

FIG. 1.19 Basic optical principle governing the operation of an optical microscope (a) the geometry on which the resolving power d of a microscope is based (b) detail showing how light from both sources must be intercepted by the lens to become part of the image. 

A.29.6 The numerical aperture (NA) is a measure of the number of orders of diffraction that can be captured by a lens, NA = n sin u. Since the image information is carried in all of these orders of diffraction, the wider the collechon angle u, the greater the magnitude of NA and the higher the resolving power, i.e. the smaller the distance between two objects set of Airy discs needed to visually separate them. 

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