By using iris diaphragm objectives the numerical aperture of the objective can be changed. If the iris diaphragm is closed, the numerical aperture is reduced. If the iris diaphragm is opened again, the numerical aperture is increased. An objective can also be used as a darkfield objective by narrowing the aperture.
Iris diaphragm objectives are labeled with the range within which the numerical aperture can be adjusted, for instance 1.4 – 0.7.
The physical relationship between aperture, resolving power and depth of field is shown in the graph. A small aperture produces low resolution but large depth of field. A high aperture means better resolution but less depth of field.