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In microscopy, the numerical aperture, AN, of an objective is:
<math>A_N = I \sin \frac {a} {2}<math>
where I is the index of refraction of the medium in which the lens is working (1.0 for air, up to 1.56 for oils), and a is the angular aperture of the lens. It is basically a measure of the diameter of the aperture compared to the focal length. In photography, the f-number expresses the same relationship.
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