Caution
Any surface or group of surfaces can be inverted except the object and the observation surfaces.

However, there are cases where ray propagation through the entire system is not possible after inversion. For instance, the inversion is refused if some edge clashes appear after inversion.

Even if it takes into account many parameters (like eventual tilts or decenters,...) in order to allow as far as possible the rays propagation through the entire system, the reversion process does not evaluate the ray propagation feasibility. In the case where the ray propagation is not possible, it may be necessary to have a look at the tilts/decenters parameters and modify them if necessary.

Note that the distance fom the object surface and to the observation surface may be redefined after inversion as well as the aperture, the fields and some options. For instance, once inverted, a focal system originally designed for an object at infinity may request to locate the object surface in the front focal plane, to enter fields in mm instead of degrees (fields table) and to select "at infinity" instead of "at finite distance" in the mode cell of the options table .