Quote Originally Posted by E. von Hoegh View Post
Ocular, not optical. Ocular = eyepiece; a small piece to go over the eyepiece of an automatic camera (when it isn't covered by your eye) to prevent light getting in and causing incorrect metering.
Some cameras have a shutter that can be moved in and out of place via a small lever on the camera body. The AE Prism finder on my Mamiya 645 Pro has this. The only use I'm aware of for it, though, is when using automatic exposure shooting off a tripod, and on a tripod I use manual settings almost exclusively. I don't think I've ever actually used one of these, of either type. Just goes to show - one person's "I have them but have never used them" is another's "must have."