The iris diaphragm is the most important single control of any optical system. This should not be used to regulate the amount of light - use the light intensity control to adjust the brightness. The iris diaphragm is the resolution verses contrast control. It does this by varying the size of the numerical aperture of the objective lens. It also controls the depth of focus.
Camera lenses have the iris diaphragm built into the objective lens.
In a microscope objective, the iris diaphragm would be so tiny that it would be hard to manufacture. The eyepiece is basically a projection lens system. The most common type is the Huygenian . This eyepiece is used with low to medium magnifications, and is designed to project the image into a human eye. These provide superior image quality.