"Pulling Focus" is, quite simply, changing the camera's focus. It's more than just changing it between shots, however. It refers to the act of changing it WHILE the camera is running. Imagine you are zoomed in down a street and a man is walking towards you. You would have to change the focus as he's walking to keep him in focus. That's called pulling focus and most film and movie crews will have a person who just does THAT and the camera opperator doesn't touch it at all.
A "Rack Focus" is when you have 2 things in the shot and one is out of focus and the other is sharp. The "rack" is when you swiftly change from one to the other. As an example, a nature documentary may have a leaf close up and sharp and suddenly the focus changes and the blurry background sharpens into a lion or something as the leaf becomes blurry.
Actually, you don't have to have 2 items, that's just a common method. Technically you could simply have a shot of an out of focus lion that you then bring into focus. That's a "rack focus" too but usually when someone talks about racks they're talking about having 2 objects in the shot.
ALL of these things are hard to do with any video camera and very VERY difficult to do with small, mini-DV cameras. We all sure try to do them a little bit, but they're much easier to pull off with film cameras that have larger lenses.
Thanks for the swift reply and explaining it so easily now I understand (someone tried to explain and baffled me with science (or bulls...). Just one more :) if in movies they have a seperate guy for pull focusing, what the hell is the camera operator for?? (just a thought)
"Pulling Focus" is usually a term used for the main function of a 1st AC, or First Assistant Camera position on film sets. It is a rather difficult job at times, when considering the person doing it typically doesn't even look through the lens. It is their job to measure distances, and mark proper measurements on the lens. It is not unusal for the focus ring on a lens to move merely a fraction of an inch in a scene. It is an essential duty for steadicam assistants, who must use a wireless remote system. Some camera operators are willing to pull their own focus, but in some cases, both of their hands are in use, as is the case with a steadicam operator. You can't compare the world of consumer and ENG video cameras to that of film and manual lenses... The difference in depth of field between the two is enormous, making the job essential in some cases.