The light transmitted to the eye by ophthalmic lenses varies according to the power of the lens. This relationship is presented theoretically and experimentally. The effect of contact lenses on light transmission to the eye is also calculated. Since ophthalmic lenses transmit variable amounts of light to the eye, an experiment was carried out to measure pupil size in subjects corrected for different ametropias. The variation in pupil size among subjects was large and was not related to the power of the ophthalmic lens. With an individual subject, on the other hand, variations were small. Possibly pupil size varies slightly when a patient shifts from glasses to contact lenses. The only significance in the difference of the amount of light transmitted to the eye by ophthalmic lenses as compared to contact lenses may be at the scotopic and photopic visual light threshold.