I’m trying to design a high NA objective lens.

Due to the high NA, some of the objective’s first order properties, namely the effective focal length and image-space NA, seem to be wrong, most probably because their calculated under the paraxial approximation (the documentation for EFFL and ISNA even say as much).

I tried to overcome this issue by defining the aperture as entrance pupil dimeter, and calculate the non-paraxial NA as the sine of the incident angle provided by the real ray trace in the image space. Assuming both of these are accurate, I could theoretically calculate the effective focal length as:

EFL = EPD/(2*tan(asin(NA)))

My question is whether or not is this technique valid, and is there other ways to get the non-paraxial first order properties of a high NA system?

**Best answer by Mark.Nicholson**