The term PLAN APO signifies a planapochromat, an objective of the highest correction, corrected for four colours chromatically and spherically. Such an objective, for its magnification, will have a higher numerical aperture than objectives of lesser correction. Planapochromats are the best objectives for critical resolution and colour photomicrography. Other things being equal, they usually have shallower depth of field. They are also more expensive.

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