The working numerical aperture of a microscope is the sum of the N.A. of the objective plus the N.A. of the condenser divided by 2. In order to retain more good contrast with good resolution, the condenser iris (aperture diaphragm) is usually opened to about 3/4 of the N.A. of the objective. Condensers also vary in their colour and spherical correction - from the relatively modest correction of an Abbe condenser to the highest correction aplanat-achromat condenser. The aplanat-achromat condenser, with its high N.A., is the best choice for colour photomicrography. Some condensers have a swing-out upper element. With this element out of the light path, the condenser is able to fill the field of view, without vignetting, of a 4x or a 2x objective.

---