1-4 Grades of Objectives

Armitage discusses grades of objectives and how aberrations are corrected. (i) A planachromat corrects for flatness of field, so the entire field is in focus from edge to edge appearing flat. (ii) A fluorite objective is highly corrected for the transmission of light. It is made of fluorite glass that doesn't absorb so much of the light. (iii) Chromatic aberration is caused by the various colors of light having different wavelengths, so without correction after magnification they would focus at different distances. An objective that corrects for all three problems would be a planapochromat fluorite objective. Not all objectives are interchangeable. Old style fixed tube length objectives with a Royal Society thread won't work on modern microscopes. Realize that not all wet objectives are for oil. Alternatively, water immersion or glycerol immersion may be specified. Numerical aperture is the strength of the objective at bringing light into the optical path to resolve your image. The best numerical aperture has the shortest working distance.