It is learned from zemaxmanual that FFT MTF is calculated from FFT PSF or FFT LSF, while FFT PSF/LSF is calculated from pupil data. As pupil sampling 2(64x64) is concerned, we can get 128x128 PSF data or 1x128 LSF data on image. When I deal these PSF/LSF data with FFT method directly, I find two problems:
- Only 64 MTF data with different frequency can be get. However, there are more MTF data in FFT MTF vs frequency curve. Does Zemax perfrom a curve fitting with PSF/LSF data before FFT or a curve fitting with MTF data after FFT?
- At the same frequency, MTF value get by directly FFT with FFT PSF/LSF data is always higher than MTF value get by FFT MTF curve. And with frequency rising, the gap will be larger. What’s the reason?
