Gaussian quadrature integration for rms spot size or wavefront error:
> Forbes, G. W. (1989). Optical system assessment for design: numerical ray tracing in the Gaussian pupil. Journal of the Optical Society of America A, 6(8), 1123. https://doi.org/10.1364/josaa.6.001123
In general, you'll want to look at MTF calculation (look at Zemax's manual for explanation/how-to). There is also a technique to target optimization at particular spatial frequencies:
> K. E. Moore, E. Elliott, et. al. "Digital Contrast Optimization - A faster and better method for optimizing system MTF," in Optical Design and Fabrication 2017 (Freeform, IODC, OFT), OSA Technical Digest (online) (Optical Society of America, 2017), paper IW1A.3
> Forbes, G. W. (1989). Optical system assessment for design: numerical ray tracing in the Gaussian pupil. Journal of the Optical Society of America A, 6(8), 1123. https://doi.org/10.1364/josaa.6.001123
In general, you'll want to look at MTF calculation (look at Zemax's manual for explanation/how-to). There is also a technique to target optimization at particular spatial frequencies:
> K. E. Moore, E. Elliott, et. al. "Digital Contrast Optimization - A faster and better method for optimizing system MTF," in Optical Design and Fabrication 2017 (Freeform, IODC, OFT), OSA Technical Digest (online) (Optical Society of America, 2017), paper IW1A.3