Extension of the aperture notion

Abstract

Elementary optics teaches us how to calculate the position and shape of an image after it has passed from an object through an optical instrument such as a magnifying glass or a microscope. To do this we use the laws of geometrical optics, and assume among other things that the image of a perfect point viewed through a centred optical lens system will itself be a perfect point. We lay down a set of conditions such as Gauss’s approximation, whereby light beams have little slope relative to the axis and are very close to it, so that we can then ignore effects such as aberration.