The Modulation Transfer Function is an important parameter in optical system design. ZEMAX offers several algorithms for computing it.

• Fraunhofer-theory based FFT methods are the most common approach. ZEMAX computes the MTF for all spatial frequencies supported by a lens (though it will optionally only display those frequencies of interest) in a graph of MTF versus frequency. When optimizing or tolerancing on MTF, only the spatial frequency of interest is computed, which is much faster and requires far fewer rays for a given level of precision. The grid method of computing MTF is available as an option, if required.
   
• For conventional optical systems with large aberrations the Geometric MTF computes an approximate MTF with great speed. It is a good choce for "roughing in" a design for best MTF as fast as one can optimize RMS spot radius.
   
• For systems which do not meet the assumptions of Fraunhofer theory, or in which the chief ray cannot be traced, the Huygens' calculation is available, and this provides a robust calculation of MTF. The only disadvantage of the Huygens method is speed.