How does a substage condenser function?
You determine the working NA of a microscope by adding the NA of the objective to the NA of the condenser and then dividing by 2. In order to retain the best balance of contrast and resolution, you usually want to keep the iris aperture diaphragm of the condenser open to about 3/4 of the NA of the objective. Condensers vary in their color and spherical correction-from the modest correction of an Abbe condenser to the highest correction aplanat-achromat condenser (which, with its high NA, is the best choice for color photomicrography). Some condensers have an upper element that can swing out of the light path, allowing the condenser to fill, without vignetting, the field of view of a 4X or 2X objective.
