a pair of cylinder lenses have a combined power of zero when their cylinder axes are parallel and a small cross cylinder power when one lens is rotated until its axis is perpendicular to the cylinder axis of the other lens . cross - cylinder lenses are used to determine the accuracy of the cylinder power and cylinder axis selected by the practitioner . by mounting one cylinder lens in a motor - driven ring - like lens mount having a member which engages another ring - like lens mount , a single motor can be used to drive both lenses ; select between parallel cylinder axes and crossed cylinder axes ; and rotate the crossed axis lenses as an assembly to the desired position .