Plan Vs Acromatic Objective Lenses

"Plan" means they are flat-field, so that the entire field is in the same focal plane. Some non-plan objectives will have good coverage such that they are sharp edge to edge but at different focal planes. These are not good for single images but are good for focus stacking. It's sometimes hard to tell if a non-plan objective has broad coverage or not since the center and edges are never in focus simultaneously. Also, some plan objectives have poor coverage, so that they lose resolution at the edges. This appears as if they are non-plan, with the edges out of focus, but in fact the edges never focus properly at any focal plane due to the poor coverage. If you plan to stack, then a non-plan objective with good coverage is just as good as a plan objective with good coverage, though the plan type may result in fewer images required in the stack.