I have set up an optical system where the light passes through the center of the optics.

When I decenter the optical system (in the +Y direction, for example) the light beam doesn't appear to change - it is not decentered in the -Y direction as I would expect. What is wrong?

Synopsis

Decentering an optical system with respect to object space illumination

Solution

Keep in mind that the light coming from object space is aimed at the center of the aperture stop in your optical system. When you decenter the optical system, you might also be decentering the aperture stop. If this is the case, the light entering your system will be decentered also. Try decentering the aperture stop surface with respect to the rest of your optical system.