Modulation Transfer Function (MTF) is an important method of describing the performance of an optical system. A consequence of applying Fourier theory to image forming optical systems, MTF describes the contrast in the image of a spatial frequency presented in the scene being viewed. See this article for more information on what MTF is.

MTF describes the imaging of the system, but an important system parameter is usually neglected: the resolution of the detector. If the detector's pixels are significantly bigger than the resolvable spot size, the optical system is said to be detector limited, and the overall system MTF is reduced compared to the MTF the optical system itself is capable of achieving.

Experimentally MTF can be measured by imaging a small bar (or single-frequency sine) chart through the lens and onto the detector. The bar chart must be small because the optical transfer function of the lens should not vary significantly over the target pattern. Within ZEMAX we can use the same method: the diffraction image analysis analyis feature is used to image a small bar chart through the system onto a pixellated detector, and the MTF is computed directly from this.