“Best form” lens: a spherical lens where the radii of the lens are optimized for best performance. For monochromatic laser lenses this involves optimizing the lens for minimum spherical aberrations. These lenses could then be either meniscus, biconvex, or biconcave. There could be very rare instances where the best form lens could be plano-spherical in shape.
The concept of the Rayleigh criterion for resolution was first introduced by Lord Rayleigh (John William Strutt) in his 1879 paper, “On our perception of small angles,” published in Philosophical Magazine. He used the criterion to explain the limits of resolution of the human eye. The paper was later published as “On the Theory of Vision” in Royal Society Philosophical Transactions. In this paper, Rayleigh described the criterion as the condition under which two close point sources of light will be seen as separate by the observer. He also showed that the criterion can be used to determine the minimum angular resolution of an optical instrument.