Based on the diagram at the right, the image-space numerical aperture of the lens is:

thus , assuming normal use in air .

The approximation holds when the numerical aperture is small, but it turns out that for well-corrected optical systems such as camera lenses, a more detailed analysis shows that is almost exactly equal to even at large numerical apertures. As Rudolf Kingslake explains, "It is a common error to suppose that the ratio [] is actually equal to , and not ... The tangent would, of course, be correct if the principal

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