The warpage of CR-39 ophthalmic spherical lenses of a variety of powers and compressed in the radial direction is measured as a function of the applied tangential force. The lenses are cut with circular shape and then mounted on a frame specially designed to measure the tangential force by means of a calibrated coil spring. The measurements are performed with a high resolution micrometer at different points on the lens surfaces. The warpage of the two surfaces of each lens is calculated and then a quantitative value of surface power variation is given. From these data, vertex power and oblique astigmatism variation are shown. Departure of the surfaces from sphere as a function of the applied tangential force is analysed.