Retired U.S. Air Force Chief Master Sgt. Mark Flockhart selects a lens to insert into a peripheral vision-testing machine at Langley Air Force Base, Va., Nov. 10, 2014. Prior to volunteering at the USAF Hospital Langley optometry clinic, Flockhart volunteered at the dental, dermatology and orthopedics clinics. (U.S. Air Force photo by Airman 1st Class Devin Scott Michaels/Released)