The substage condenser of a microscope is design to focus the light onto the specimen

The substage condenser of a microscope is design to focus the light onto the specimen. In addition it must also fill the numerical aperture of the objective. Like objective lenses there are several different types. The most common being the Abbe condenser. This type is not corrected for optical aberrations. The achromatic condenser is corrected for both spherical and chromatic aberrations. Both types of condenser have their numerical aperture printed on the side. This needs to be of equal or greater value then that of the objectiveN.A., otherwise, the full resolution of the objective will not be utilized. Most substage condensers can use immersion oil like that of the objectives to achieve their full N. A. This is not recommended unless you are doing very demanding photomicroscopy work.

The condenser aperture diaphragm is responsible for controlling the angle of the illuminating light cone and, consequently, the numerical aperture of the condenser. This concept is illustrated in Figure 3, where a series of condensers are illustrated with light cones (and numerical apertures) of decreasing size from left to right
in the figure. Condenser light cone shapes are also a function of the degree of correction for optical aberration.

It is important to note, with respect to the size and shape of condenser light cones, that reducing the size of the field diaphragm only serves to slightly decrease the size of the ower portions of the light cones illustrated in Figure 3. The angle and numerical aperture of the light cone remains essentially unchanged with reduction in field diaphragm size. Another important concept, often overlooked by novices, is that the intensity of illumination should not be controlled through opening and closing the condenser aperture diaphragm, nor by shifting the condenser axially with respect to the
optical center of the microscope. Illumination intensity should only be controlled through the use of neutral density filters placed into the light path or by reducing voltage to the lamp (although the latter is not usually recommended, especially for photomicrography). To ensure the maximum performance of the tungsten-halide lamp, refer to the manufacturer's instrument manual to determine the optimum lamp voltage (usually 6-10 volts) and use that setting. Brightness of the illumination can then be easily controlled by adding or removing neutral density filters.