Levels of Color Correction

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Microscope objectives have varying corrections for chromatic and spherical aberrations that arise during magnification.

Common Objective Color Corrections
Type Chromatic Aberration Correction Spherical Aberration Correction
Achromatic Blue, Red Green
Fluorite or Semi-APO Blue, Red (Green is close) Blue,Green
APO Deep Blue, Blue, Green, Red Deep Blue, Blue (sometimes green)

Objectives with higher color correction typically also have a higher numerical aperture, which allows them to resolve finer details.

"Plan" objectives are those that attempt to have low aberration due to field curvature for most of the image. An objective that is not plan will have a flat field in the center 65% of the image, with some blurring around the outside. Semi-plan objectives have a flat field over about 80% of the viewable field. Plan objectives are flat over about 90%. When viewing through the objective, field curvature aberrations are not always bothersome, but they tend to be more important when photographing