The performance of high-resolution objectives is optimal when the refractive indices of the specimen and all intermediate optical media match the values for which the objective is designed. Linear correlation between aperture and resolution (green), respectively exponential correlation between aperture and depth of field (red) A high aperture means better resolution but less depth of field. A small aperture produces low resolution but large depth of field. The physical relationship between aperture, resolving power and depth of field is shown in the graph. Iris diaphragm objectives are labeled with the range within which the numerical aperture can be adjusted, for instance 1.4 – 0.7. An objective can also be used as a darkfield objective by narrowing the aperture. If the iris diaphragm is opened again, the numerical aperture is increased, the resolution is increased, but the depth of field is decreased. If the iris diaphragm is closed, the numerical aperture and resolution are reduced but the depth of field is increased. This is especially useful for widefield microscopy. By using iris diaphragm objectives the numerical aperture of the objective can be changed.
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