Compound microscope objectives are marked with the terms “Achromat”, “Semi Plan”, or “Plan” to indicate how well they correct for flatness of field,* which determines how much of the field of view (FOV) is in focus. Achromatic objectives have no correction for flatness of field and in general may have around 65% of the center FOV in focus, with the edges somewhat blurry. Semi Plan objectives have some level of correction with roughly 80% of the center in focus. Plan objectives have the best correction with 95% or more of the FOV in focus.

Note that these are ballpark estimates and the exact level of correction may differ from one manufacturer to the next. The markings themselves may be spelled differently, so instead of “Semi-Plan” it could say SM Plan or S. Plan, although they mean the same thing.

Besides flatness of field, there are other corrections to be made for various optical problems. Lenses with additional corrections include:

• Plan achromat - An objective that corrects for chromatic aberration in two wavelengths and spherical aberration in green, in addition to field curvature.
• Plan fluorite - An objective with correction for 2-3 colors for both chromatic and spherical aberration. These are ideal for fluorescence microscopy.
• Plan apochromat - Offers the highest level of correction for chromatic (4-5 colors) and spherical (3-4 colors) aberration.

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Footnote

* Also called "field curvature". But the term "flat-field correction" seems to be used in two different ways. In Microscope Objectives, Rottenfusser et al state that

"Plan achromats provide flat-field corrections for achromat objectives (Figure 2)."

They're most likely referring to correction for field curvature (and probably mean to reference Figure 3, which compares Achromat with Plan Achromat, not Figure 2).

Edmund Optics uses the term in a similar sense:

"Plan-apochromatic objectives are the most complex, high-end objective design with chromatic and flat field correction done within the objective itself."

However, Wikipedia and others use flat-field correction to refer to the removal of unevenness in brightness, not field curvature:

"Flat-field correction is a technique used to improve quality in digital imaging. The goal is to remove artifacts from 2-D images that are caused by variations in the pixel-to-pixel sensitivity of the detector and/or by distortions in the optical path."

From Petzval field curvature:

"Not to be confused with flat-field correction, which refers to brightness uniformity."