what does the diaphragm do in a microscope

The diaphragm in a microscope, also known as the iris diaphragm, regulates the amount and shape of the light that travels through the condenser lens and eventually passes through the specimen by expanding and contracting the diaphragm blades that resemble the iris of an eye. The diaphragm controls the amount of light that is allowed to pass through the microscope stage and up toward the objective lens, and it can be used to vary both the intensity and size of the cone of light that is projected upward into the prepared slide. The luminous-field diaphragm determines which portion and size of the specimen is illuminated, while the aperture diaphragm of the condenser is imaged on the pupil of the objective and regulates the illumination of this pupil. The primary function of the diaphragm is to change the angular aperture of the cone of light that is produced after the light travels through the condenser, and the size of this cone of light is important because if there is a mismatch between the size of the cone of light and the optimal numerical aperture on the objective lens in place, the optimal image quality will not be obtained. Therefore, the diaphragm is an important component of the microscope in focusing the incoming light and improving the quality of the image.