Hi Erin,

Thanks for your post on our forums!

For the Image Simulation tool, one of the components of the analysis is the computation of a PSF Grid, which evaluates some array of PSFs across the field of view as defined by your input JPG/BMP/etc file. This PSF Grid is computed through real ray tracing results, which begins at your Object surface. So, if your Object surface is curved, then the off-axis field points will begin their ray trace in generating the PSF Grid at whatever Z-offset there is from the curvature of the Object surface. We can see this in the PSF Grid results from Image Simulation for the following system:
 

When Image Simulation performs the convolution step, OpticStudio will convolve the PSF Grid shown above with your input image file (so, in Config 2, the off-axis points will degrade in image quality due to the broadened PSFs):
 

Let us know if this helps clarifies things or if you have any more questions here!