I've seen a convention for calculating "magnification" of a loupe as: 250/f + 1 where f is the focal length measured in millimetres. As Emmanuel BIGLER points out the 250/f formula is also used. I think which formula applies depends on whether the magnifier is used very close to the eye, add +1, or away from the eye, don't add +1. The dioptre power of the eye itself is a confounding factor and the magnification marked on a loupe is but a guide.
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