Condenser. ï An adjustable lens free of spherical aberration and coma is placed in front of the light source to focus the light at the desired point in the optical path. ï A field diaphragm is placed in front of this lens to minimize internal glare and reflections within the microscope. The field diaphragm is stopped down to the edge of the field of view. ï A second adjustable-iris diaphragm, the aperture diaphragm, is placed in the light path before the vertical illuminator. ï Opening or closing this diaphragm alters the amount of light and the angle of the cone of light entering the objective lens. ï The optimum setting for this aperture varies with each objective lens and is a compromise among image contrast, sharpness, and depth of field. ï Opening this aperture increases image sharpness, but reduces contrast; closing the aperture increases contrast, but impairs image sharpness. ï The aperture diaphragm should not be used for reducing light intensity. It should be adjusted only for contrast and sharpness.