A condenser is an optical lens or lens system located within or below the stage (sub-stage) on compound microscopes. Its primary function is to gather the light coming in from the illuminator and to concentrate that light into a light cone onto the specimen. The condenser and diaphragm of the microscope work in conjunction with each other to illuminate the specimen to be seen clearly. The size and numerical aperture of the light cone is determined by adjustment of the aperture diaphragm. The condenser performs several key functions:

• Distributing light evenly over the specimen to remove lighting imperfections .
• Aberration correction .
• Controlling the angle of the cone of light projected into the objective lens above .

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. Without a condenser, the microscope would not focus the light emanating from the source correctly.