The Nyquist frequency, named after the Swedish-American engineer Harry Nyquist or the Nyquist–Shannon sampling theorem, is half the sampling frequency of a discrete signal processing system. It is sometimes known as the folding frequency of a sampling system.
The sampling theorem shows that aliasing can be avoided if the Nyquist frequency is greater than the bandwidth, or maximum component frequency, of the signal being sampled.
For example, Gatan camera MTF at 1/2 Nyquist frequency is about 0.1 at 200keV.
Note that, 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 (also called reciprocal pixel size). In other words, it is impossible to separate two objects in an image if they are closer than one pixel away from each other.