Achromatic vs Semi-Plan vs Plan
Besides some special microscope objective lenses, there are three kinds of objective lenses on the market.
Achromatic, semi-plan, and plan.
First of all, all these three objective lenses correct for color, although there's no "achromatic" in the name of "semi-plan" or "plan". The difference between these three lenses is the focusing area which can be seen from the eyepieces when using high power objective lenses such as 40x, 60x and 100x.
The difference from low power lenses are not obvious, which means that can be ignored. At high power, an achromatic lens has about 65% of focused area acrossing the center. A semi-plan one has about 85% whereas the plan one has 95%.
Due to the correction from the different lenses, the price for those lenses are also different. Here are some links for the objective lenses: