What is it?

[Bsmith]

I was wondering about what something is that I see most big time directors using...its this medium sized scope...looks like a lens...I always see the director looking through it...I was wondering what it is...what its used for...and where do you get one if its affordable.

 

[Lazlo]

Those are for previzualization. The director can look through it, to get a feel for framing his shots. I am not really familiar with them, but there are companies that just make those. I have a very strong feeling (though I'm not positive) that they are very expensive. You really don't need one. YOu can just look through your own camera lense and get a feel. Their cameras are a lot bigger though, and it is harder to frame for shots with a big 35mm camera that can only be moved smoothly via track, dolly, crane, steadicam.

 

[Digigenic]

8)
The technical term for the scope you are referring to is called a viewfinder, typically used between directors and cinematographers to compose a shot.

As Lazlo said, you can look through the viewfinder that's available on the camera to accomplish the composition of a shot.
But, when used on sets that employ larger camera systems, a mobile viewfinder is a necessity. In those circumstances, a viewfinder ensures a greater a level of accuracy in the outcome of the shot, because it allows the director and the DOP to collaborate in the pre-visualization process as they strategically scale the next shot.

Mini viewfinders, found on sites like filmtools.com can cost a few hundred dollars. Where as, the viewfinders that you've seen directors and their DOP's meticulously peer through on set for big budget movies are priced considerably more.