The function of the condenser in a microscope is to gather light from the microscopes light source and concentrate it into a cone of light that illuminates the specimen with uniform intensity over the entire viewfield). The condenser is located beneath the stage on upright microscopes and above the stage on inverted microscopes). The size and numerical aperture of the light cone is determined by adjustment of the aperture diaphragm). The condenser height is controlled by a rack and pinion gear system that allows the condenser focus to be adjusted for proper illumination of the specimen. The numerical aperture of the condenser should equal or exceed that of the objective lens, and higher powers will need condensers with higher numerical apertures. The Abbe condenser is the most widely-used type of condenser for bright field microscopy. It is composed of two or three lenses, and the upper short-focus lens can usually be flipped out of the optical path or unscrewed in order to fill the field of view with low power objectives. The condenser also provides an area of evenly-illuminated light in the field of view at the specimen plane and illuminates the aperture of the objective uniformly with light of sufficient yet controllable angle. The condenser has to be focused properly in order to achieve the best quality image, and this is true whatever method of contrast enhancement is used.