I am trying to find the best focal plane of an optical system with two lenses. The distance between the last lens surface and the imaging plane is varied to minimize RMS spot size or POPD, data=51, Xtr1=0.86. The result I got indicate I will get a focus at ~57mm after the last lens surface with ray tracing, but with POPD, it seems I will never get a focus after the last lens.

I am wondering what could be the cause of such a big difference and which is more trust-worthy? The ray tracing result makes more sense, but shouldn’t POP give more accurate results?

What am I missing here?

Look forward to your advice. 

Thanks very much.