Interpreting Seidel Aberration coefficients

[NazRB]

I am trying to model a simple system, but the ray-tracing does not seem to be consistent with the analysis of the system in terms of Seidel aberration values. Here's the system layout:

When the system contains only the Eye model and the OL lens, it can be referred from the Seidel diagram that the SUM of the aberrations at IM1 for field curvature and distortion are +9um and -19um respectively. It can seen that the peripheral rays' focus is lagging the paraxial focus - thus, the image is not flat:

Then, when I add the achromatic pairs P1 and P2, the new image is at surface Im2, the new SUM of Seidel coefficients for filed curvature and distortion are +13 and -20 respectively:

Thus, I would expect the IM2 surface to be even more curved, however, looking at the IM2 plane ray diagram, the peripheral points seem to focus much closer to the paraxial image plane - thus, the field curvature seems to be much smaller. So, the question is why the Seidel coefficients for filed curvature for IM2 are larger than for IM1, but the apparent ray-tracing shows smaller filed curvature in IM2 (which is expected to be larger than in IM1)?

 

[gtoguy97]

A:The Seidel coefficients are based on a paraxial approximation. The ray tracing, on the other hand, shows what is actually happening in the system. The difference between these two results could be caused by different factors. If the system is not designed to be telecentric, the entrance pupil shape and size can affect the field curvature. Also, since the Seidel coefficients are approximations, they can also be affected by other aberrations that are not included in the approximation. Finally, the materials used for the lenses can also affect the actual ray tracing results compared to the Seidel coefficients.