Practical Electron Microscopy and Database

Assuming that there is sufficient illumination, the smallest distance between two points that human eyes can resolve is about 0.1 - 0.25 mm, depending on how good their eyes are. This distance is the resolution or resolving power of our eyes. Therefore, any instruments that can show us pictures (or images) revealing details finer than 0.1 mm could be called as a microscope, and its highest useful magnification is governed by its resolution.

On the other hand, to avoid having the overall CTF dampened too much by spatial coherence envelopes, temporal coherence envelope function and detector envelope function, it is important to find and use an optimum magnification (the highest useful magnification) since it is impossible in a pixel image to detect spatial frequencies less than the Nyquist limit. In other words, it is impossible to separate two objects in an image if they are closer than one pixel away from each other.