The Cavision RFSC-4 is a follow focus crank lever used to perform quick and deliberate focus racks. The lever easily attaches to the inside core of the focus knob and perpendicularly extends outward. The crank lever is especially useful for assistant camera operators by comfortably giving them maximum control.
I bought this to go with my Smallrig follow focus, because Smallrig doesn't offer a crank and I thought that both were the standard 12mm square. With a little force, I can get this in just far enough to stay in place, but the retaining ball is in the wrong position to work with the Smallrig FF. It works well enough for now. Eventually I'll try it with another FF and try another crank with Smallrig to figure out which one is right, then use a small file or dremel to adjust whichever one is the culprit.
Its well made a precisely machined, but still seems expensive for what it is. But it gets the job done and is compfortable to use.