What are the differences among achromat, fluorite and apochromat objectives?
Achromat, or planachromat, objectives provide correction for 3 wavelengths chromatically and one or two wavelengths spherically in the middle of the visible spectrum. They give their best images in green light, and in white light will yield satisfactory (but not the best obtainable) images for color photomicrography. Fluorite, or planfluorite, objectives provide correction for four wavelengths chromatically and spherically over a correspondingly wider spectrum than achromat objectives serve. Fluorite objectives yield good and relatively economical images for color photomicrography. Apochromat, or planapochromat, objectives, for their respective magnifications, have a higher NA than objectives of lesser correction and deliver the highest degree of correction for four wavelengths chromatically and spherically. These are the best objectives for critical resolution and color photomicrography, and they usually have shallower depth of field than the other objectives. They are also more expen
