Maj. Bryan Kemper, 28th Medical Operations Squadron optometrist, uses a phoropter to test Master Sgt. Juan Lopez, 28th Aircraft Maintenance Squadron production superintendent, for vision deficiencies at Ellsworth Air Force Base, S.D., Sept. 12, 2013. Optometrists use phoropters to measure the refractive status of each eye and determine the correct prescription. (U.S. Air Force photo by Airman 1st Class Alystria Maurer/Released)