The cut end of E.B. Mayer's hair magnified 1000 times in an electron microscope. The horizontal line indicates a length of 20 microns. In practice it is how small are the details you can see with the lens. Resolving power is an instrument property specifying the smallest detail that a microscope can resolve in imaging an ideal specimen. Resolution refers to the detail actually revealed in the image of a real specimen.
High resolution refers to small values of the minimum resolvable distance.
Of course, this limit requires ideal lenses under ideal conditions. Depth of focus is the distance above and below the geometric image plane within which the image is in focus, Fig. 2. As one goes to higher and higher magnification, the depth of field in the sample gets smaller and smaller. It becomes hard to keep the entire specimen in focus. The compound microscope, Fig. 3, has two or more lenses.