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By using iris diaphragm objectives the numerical aperture of the objective can be changed. This is especially useful for widefield microscopy. If the iris diaphragm is closed, the numerical aperture and resolution are reduced but the depth of field is increased. If the iris diaphragm is opened again, the numerical aperture is increased, the resolution is increased, but the depth of field is decreased. 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.