Koehler illumination, invented in 1893 by August Koehler, avoids flares and ghosting, and is essential for activities such as microscope photography.
The field stop is used to limit the area of illumination on the specimen. Limiting the amount of light cast helps eliminate flares and ghosting, leading to sharper images.
The aperture diaphragm limits the projection of the light source (filament), which enables adjustment of brightness of the field of view. The angle of incidence may also be adjusted with the aperture diaphragm, which allows adjustment of contrast resolution and focal depth. Focal depth refers to the depth of thickness within the specimen that comes into sharpest focus.