Why cant I set a thickness on the image surface?
ZEMAX uses the convention that the thickness of a surface is the distance to the next surface. Since the image plane is the last surface, there is no next surface. Although some programs use this thickness as a defocus term, ZEMAX does not. The reason is primarily to keep the interface consistent, and of course because ZEMAX does not force the image plane to be at paraxial focus in the first place. The overhead of using a dummy surface before the image plane for the defocus term is insignificant.
