This article describes how to interpret the sampling used in wavefront-based analyses.
When doing numerical computations, we want to preserve pupil symmetry, and we want the chief ray to be an actual point in the center of the array. Also, for calculating the FFT we need to center the array on the center of what the FFT routine expects. To do all this, we define the center of the pupil in exit pupil space (the near field, or the spatial domain, depending upon your perspective) as (n/2 + 1, n/2 + 1).
To determine the frequency spacing in the FFT MTF surface plot, OpticStudio uses the fact that the edge of the autocorrelation is at 1/(λ*F/#), where λ is the shortest wavelength in the system (if we ask for the polychromatic result).
Reference Source: https://www.zemax.com/
