It’s because it is simply the Fourier transform of the pupil function. It doesn’t know where your image plane is, nor how it is oriented. The Huygens calculation is a rigorous calculation done on the image plane itself, so it is superior in this regard (and in all regards, except speed, though on modern machines I don’t think speed is such an issue any longer.

The golden rule: any differences between FFT and Huygens, trust the Huygens as long as it is adequately sampled such that increasing or decreasing the sampling does not make any significant difference. 

• Mark

Hi Mark

Thanks for your reply.！Your answer is great ! Then I still have another question： If we are not sure about the direction of the FFT diffraction surface, why do we still use the optical path data of the plane perpendicular to the chief ray when tracing the reference sphere?

Best，Nancy

Hi Nancy

Do you mean when plotting OPD or wavefront map?

In our software, the OPD and wavefront map used the reference sphere based on the chief ray. It is a quick way to assess the quality of the optical system. But then when we optimize, we actually reference the wavefront to an average sphere. This is what we call the centroid reference. 

Sandrine