An annular diaphragm and an iris diaphragm both adjust the amount of light that passes through a specimen in a microscope, but they do this in different ways. In an iris diaphragm, a lever or wheel is used to adjust the positions of a set of moving plates (sometimes referred to as "leaves"). As the leaves shift position, a hole in the center of the leaves is made smaller or larger. The size of this hole controls how much light passes through to the specimen.
An annular diaphragm consists of a plate with a series of holes of different sizes in it. The microscope user turns the plate to line one of the holes up with the light path. The size of the hole chosen determines the amount of light that passes through.