There are numerous types of objectives; several are listed below:
1. An aplanat is free from spherical aberration and coma.
2. A plan objective has a flat field: there is no Petzval curvature.
3. An achromat is corrected at two wavelengths.
4. An apochromat is corrected at three wavelengths.
5. A semiapochromat is nearly corrected at three wavelengths.
The original fluorite objectives corrected for color with fluorite glass, but lanthanum glass eventually replaced fluorite in most applications. A modern fluor objective contains LaK while excluding many schwer flints (SF). A fluor objective transmits well in the UV; consequently, a fluor objective may also imply application to fluorescence.
The magnifications of the following objectives are based on a 200-mm tube lens, which is a standard tube lens for Nikon microscopes. An Olympus scope employs a 180-mm focal length as its tube lens.
12.2 Aplanatic Surface
An aplanatic surface is free of both spherical aberration and coma. There are three configurations of an aplanatic surface. An aplanatic surface of the flat kind has little practical value: the object and image are located at the surface. An aplanatic surface of the concentric kind is very intuitive: the object and image are concentric with the surface. There is no refraction in the second configuration. An aplanatic surface of the scaled-divergence kind is not obvious: it is defined by the following mathematical relation, where the divergence is scaled by the refractive index:
NAI / nI = NAO / nO .