Semi-Plan Achromat Objectives - these are one step up from basic microscope objectives in that the field of view (circular image) has been corrected a bit and the outer edges will be in focus more than the standard achromat lens. However, the entire field of view will not be a completely flat and focused field - the very outer edge will still be a bit out of focus.
Plan Achromat Objectives - these microscope objectives have been corrected for lens curvature and the entire image plane is flat and in focus - including the edges of the image.
Plan Apochromat Objectives - these microscope objectives are also referred to as Plan Apo. These objectives have been corrected for four colors chromatically and spherically. Plan apochromat objective lenses are the best objectives to use for critical resolution and color photomicrography. These objectives typically have a shallower depth of field.
Plan Fluorite Objectives - these microscope objective lenses are corrected for four wavelengths, but not quite as extensively as plan apochromat objectives are. Plan fluorite objectives are well suited for photomicrography and are typically a bit less expensive than plan apochromat objectives.
If you are looking for a specific type of microscope objective lens please email Microscope World for further information.