ABSTRACT
A lens is subjected to various tests for acceptability after manufacture. These tests can either be most rigorous or rather cursory. It is therefore prudent and necessary for the purchaser or end user to devise or have available some means of evaluating a lens or optical system. Indeed, ‘infinity’ for many optical purposes is taken as a distance greater than 20 focal lengths, but 1000 f is a more sensible value. Large-diameter collimators are best constructed using reflective optics such as off-axis paraboloids instead of refracting lenses. The simple collimator may be refined by the addition of a beamsplitter plate adjacent to the target and with a graticule in an equivalent focal plane, viewed by a high-power eyepiece. A collimator must not be used to give an artificial infinity. A magnifier or low-power microscope is used to observe the point spread function image in the focal plane of the lens under test.
