The diaphragm on a microscope is typically located beneath the stage, where it controls the amount of light that passes through the sample and up the microscope tube to the eyepiece(s) or camera. It is used to adjust the brightness and contrast of the image seen through the microscope.

Some microscopes may have multiple diaphragms, such as a field diaphragm and an iris diaphragm. These diaphragms can be adjusted to control the size and shape of the light beam as well as the amount of light that passes through it, allowing for more precise control of the image quality and visibility of the sample's features.