Use the spot diagram when:

• The geometric aberrations are large compared to the diffraction effects. This can be checked using the "Airy Disk" scale option on the spot diagram.

Use the FFT PSF when:

• The chief ray makes a small angle with respect to the image surface normal.
• The exit pupil is not significantly distorted with respect to the entrance pupil.
• Speed is more important than absolute accuracy.

Use the Huygens PSF when:

• Maximum accuracy is desired.