The condenser is an important part of a compound light microscope that helps focus the light through the sample and the objective lens. It is a glass lens or lens system located within or below the stage (sub-stage) on compound microscopes. The basic function of the condenser is to gather the light coming in from the illuminator and to concentrate that light into a light cone onto the specimen. The condenser is located above the light source and under the sample in an upright microscope, and above the stage and below the light source in an inverted microscope). The aperture and angle of the light cone must be adjusted (via the size of the diaphragm) for each different objective lens with different numerical apertures). The key features of the condenser are:

• Numerical aperture: A condenser’s numerical aperture (measure of a microscope lens’ resolving power) should equal or exceed that of the objective lens. Higher powers will need condensers with higher numerical apertures.

• Fixed and moveable: A basic condenser is fixed in place. 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 system.

• Aplanatic-achromatic condenser: The highest level of correction for optical aberration is incorporated in the aplanatic-achromatic condenser. This condenser is well corrected for both chromatic and spherical aberrations and is the condenser of choice for use in critical color imaging with white light.

To use the condenser, the aperture and angle of the light cone must be adjusted for each different objective lens with different numerical apertures). The size and numerical aperture of the light cone is determined by adjustment of the aperture diaphragm. A microscope condenser will operate optimally when the light intensity of the microscope is also set accordingly.