- Rayleigh criterion of resolving power
When a lens system with a circular aperture is free of aberrations, the image of a point object will appear as a disc of finite size surrounded by concentric rings. When two points are separated such that the center of the image of one falls on the first minimum of the second, the points are said to be resolved according to the Rayleigh criterion. This condition exists when the angular separation of the point objects, viewed from the objective lens of the instrument, is:
is the light wavelength and a is the diameter of the objective lens.