PCX Lens Orientation Note

This is a rather basic thing in the optics world, but it is sure hard to believe that just placing a lens in a beam line one or the other direction can make a difference in the imaging. 

It's easy to make a little simulation to check what really happens in the focus spot.

Let's take a simple, collimated beam with a few rays and a plano-convex lens and observe what happens in the focus in one of the other orientation.

Focal spot at the same scale, Left: Plane side of the lens facing the beam. Right: Curved side of the lens facing the beam.

It is obvious that the arrangement with the curved surface towards the collimated beam is producing a smaller spot.

The explanation is rather simple: If the flat side is facing the beam, the only point of refraction is the curved surface of the lens. If the lens is rotated 180 degrees, there will be two refractions, each with a smaller incident angle. Optical aberrations are dependent on the incident angle so the larger the angle, the larger the aberration. Replacing the lens with a plan-parallel plate would give the best results regarding aberrations, but then the system is useless for imaging. Optical engineering is full of compromises...