500 mm lens question

I just picked up an inexpensive (used), 500mm lens. I got it just to fool around with because it was cheap. But without sounding too ignorant here. What is the O<-->C dial for on the lens, and what do I do with it ? It appears to stop it down internally?
Thanks,
Rich

That's what it do! You "preset" the other ring to the f/stop you want, then turn the O-C ring from O to C to take the picture. At O the image is brighter for focus.
If you are using with a camera that has Shutter priority metering you can put the click ring to a large number, say 22, and turn the other ring until a shutter speed lights up that is appropriate for the situation.

Walt

While we're on this subject, what would this lens be used for primarily? Brian