(Left) Schematic of some CTFs and the corresponding PSFs (object-image pairs for a very small object). (A) A perfect (impossible) microscope; (B) a typical good microscope; (C) a poor or improperly used microscope. The dashed line lies at a relative contrast of 25%, corresponding to the Rayleigh criterion for the resolving power of a microscope. (Right) Images of a tiny object from a microscope at two different values of defocus reveal the 3D nature of the PSF and CTF. Objects in some size ranges appear to change from black to white or vice versa as the focus level changes between the two indicated values. An example of this contrast reversal in a transmitted light image is shown in Figure 4.