The Geneva lens measure is an instrument that can measure the curvature, power, and thickness of ophthalmic lenses. It works by having two fixed pins and a movable central pin that indicates the sag of a curved surface when placed on a lens, directly reading the refractive power. It can measure spherical, cylindrical, and bifocal lenses. For materials other than the calibrated refractive index of 1.523, a conversion factor must be applied to calculate the true power. The lens measure is an important tool in optics that has applications beyond just measuring surface power of lenses.