Eyepiece
The F-1s eyepiece was threaded and could take a metal (later soft rubber covered) ring, an eyecup or several different diopter adjustment lenses. Th Magnifier R and Angle Finders A2 and B could also be attached to allow critical focusing and / or waist level use (if one didn't want to fit either the Speed Finder or Waist Level Finder).
Read more about this topic: Canon F-1