New method for determining the magnifying power of telescopes.

A new method of measuring the power of optical telescopes is described. This method makes use of the vergence amplification that occurs when the light incident on the objective lens at a telescope is divergent or convergent. The relation between the vergence incident on the objective and vergence emergent from the eyepiece depends on the magnifying power and the length of the telescope. The method is most simple to apply to short-length, low-powered Galilean tesescopes, such as those used as low vision aids, in sports glasses, and in telescopic loupes. With such telescopes, holding a lens of known power against the objective and measuring the back vertex power of the lens-telescope combination using the lensometer and then measuring the telescope length allows the determination of the magnifying power of the telescope. The method can be adapted to determine the magnification rating of near-vision telescopic loupes.