Resolving Power

Optical instruments -- the human eye, a microscope, a telescope -- are used not just to make distant or tiny objects appear bigger (magnification), but crucially to reveal FINE DETAIL: to show two objects that are physically close together, or two small parts of one larger object, as genuinely DISTINCT from one another, rather than blurring together into one indistinguishable blob. This ability to separate two closely-spaced objects into two clearly distinguishable images is called the RESOLVING POWER of the instrument. A simple everyday illustration: two fingers held up (a 'victory sign') from within about 10 m are easily seen as two separate fingers, but the same two fingers, viewed from beyond about 50 m, can no longer be confidently told apart from a single finger -- the eye's resolving power has been exceeded. …