If you zoom in close to the edge of the stop surface in the layout, you can see that the marginal rays do not land exactly on the edge of the stop.

This is because without Ray-aiming, the rays from the objects space are aimed at the entrance pupil; the paraxial image of the stop as seen from the object space. The paraxial calculation for entrance pupil size and position considers only the power of the optics between the object and the stop surface and  not the aberration. Ideally, when ray tracing, only rays that land on the correct stop coordinates should be chosen to property sample the system aperture. For example, the very definition of marginal ray requires it to intersect the edge of the stop rather than away from it.

The pupil aberration plot shows a maximum value of 3% with Ray-aiming off, indicating that Ray-aiming may not be necessary. Users need to always consider the trade off between the increased ray trace accuracy vs. computational time, when using Ray-aiming is optional. In general, Ray-aiming will decrease the ray trace speed roughly by factor of 2  to 8.

Now, lets turn the Ray-aiming to “Paraxial” and observe the change in the layout.

The Ray-aiming algorithm iteratively finds the rays in object space that will yield the desired stop surface coordinate. The layout shows all marginal rays intersecting the exact stop semi-diameter. The aberrations are still present but they are accounted for when choosing the rays to sample the stop. Ray-aiming does not remove the pupil aberration but rather accounts for it when choosing input rays.