Microscope objectives come in many different designs. Three of the most common are achromatic, semi-plan and plan. In an achromatic objective, which is the most common type, there is one achromatic lens. Achromatic objectives correct for color and have a flat field in about the center 65% of the image. This is not to say that the outer 35% of the image will be blurry- this just means that if there are aberrations, they occur in the outer 35% of the image. A semi-plan, or semi-planar objective, is an intermediate between the achromatic and the plan objective. Semi-plan (sometimes called micro-plan as well) objectives have an 80% flat field. 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. While plan objectives give you flatter fields than achromatic objectives, they also have a higher price.
Eyepieces for both microscopes and telescopes come in varying optical designs. The major differences between the three microscope eyepiece designs we carry is listed below: