Most microscope objectives come in three basic designs: achromatic, semi-plan, and plan.
An achromatic objective corrects for color and has a flat field across the central 65% of the image. It’s usually a pair of lenses.
Semi-plan (semi-planar, sometimes called micro-plan) objectives have an 80% flat field. They can either be two lens element achromats or three or more element apochromats.
A plan (or planar) objective corrects better for color and spherical aberration than either the semi-plan or the achromatic objective. Plan objectives have a flat field about the center 95% of the image. They also often have larger working distances. While plan objectives give you flatter fields than achromatic objectives, they also are the most expensive