
Using the field curvature fan
The field curvature fan is a plot of ray intercept (where the ray hits the image surface) relative to the paraxially calculated intercept, as a function of field coordinate (object height or angle).
To create a field curvature fan window:
- Select from the menu Analysis > Fans > Field curvature

The astigmatism and field curvature plot shows the focus relative to the image plane as a function of field. For the x fan the calculation is as follows: for each field point a triplet of rays is aimed at pupil coordinates (0,0), (0.01,0), and (-0.01,0). These rays are traced through the system and the focus value is where the RMS x dispersion of the triplet is minimized. For the y fan the calculation is as follows: for each field point a triplet of rays is aimed at pupil coordinates (0,0), (0,0.01), and (0,-0.01). These rays are traced through the system and the focus value is where the RMS y dispersion of the triplet is minimized.
The pupil coordinates used are small compared to the full-pupil value of 1, so the calculation is essentially, but not precisely, paraxial.