A condenser is an important component of compound microscopes that is located within or below the stage (sub-stage) . Its basic function is to gather the light coming in from the illuminator and to concentrate that light into a light cone onto the specimen). High-power objective lenses have very tiny diameters and require concentrated light to work properly. Condensers aid greatly with the bright-field illumination most commonly used with microscopes.
A basic condenser is fixed in place, while a moveable, more precise and more expensive condenser is the Abbe condenser. It usually can be moved vertically, regulating the amount of light from the illuminator. Mounted sub-stage, it often has an adjustable iris-type diaphragm to control the diameter of the beam of light entering the lens. Correct positioning of the condenser with relation to the cone of illumination and focus is critical to quantitative microscopy and to ensure the best digital images.
The function of the condenser is not to magnify light in any way, but to manipulate its direction and angles of reflection. The condenser controls how much light from the illuminator is permitted to pass up through the aperture, controlling the intensity of the light. It also, critically, regulates the contrast. In darkfield microscopy, a special component called a condenser is employed to force light to bounce off the item of interest at such an angle that the object is easy to visualize in the same general manner as a silhouette.
