In a scary move you see sometimes a film effect were the hallway becomes longer like infinite while the actor is standing still. How do they do that?
Is that done with green screen, while changing the background?

Can you give an example so we know what you mean? Do you mean a trombone shot (well it has a million different names)? This is seen in Jaws, when Roy Scheider sees the shark attack the young boy, on Fellowship of the Rings when Frodo senses the Dark Rider approaching and they hide under the tree. It was made famous by Hitchcock in Vertigo to demonstrate an uneasy feeling when James Stewart looks down the stairs. Apparently the effect was invented for that film by one of the camera department.

It is achieved by changing zoom on the lens and dollying the camera at the same time so that the subject stays the same size in the frame. This creates an effect of the background "warping" whilst the subject does not.

It's also called a "zolly".

It's also called a "zolly".

I've never heard that one. Everyone seems to have a different name for it. I've heard one or two people call it a conta-zoom as well.

This was also discussed in the thread below.

http://www.dvinfo.net/conf/showthread.php?t=42725


Andy.

Hey cool, the "zolly" name is indeed appearing to stick. Perhaps I should have somehow appended my name to it, assured my legacy etc...!

I knew this forum would give me the answer the same day. Great.
I always wondered how do did it. Last night I saw a movie were they used it. It was done perfectly and at the right moment. Which made the moment overwhelming. Now that I know how they do it, I realize how difficult it must be to do it manually with the zoom. Mr Hitcock, my hat off for inventing this.
(My wife sometimes asks me if I'm watching the story of the movie, or watch only how they produced and made it... at least I will not be wondering around the house anymore how they did this one)

...Mr Hitcock, my hat off for inventing this...

According to imdb, it was actually the second unit cameraman Irmin Roberts:

http://www.imdb.com/title/tt0052357/trivia

Yeah, I've done it a number of times and it never gets easier. The trick that the person who operates the zoom lever has to understand is that generally there is an exponential nature to the mechanism which is that the same amount of lens rotation will have a greater and greater effect as you go wider, which requires one to ramp down the speed of the zoom as you get to the end. It's especially hard if you need to keep the beginning or end (or --shudder--both) of the move intact for the scene, as that requires the dolly grip and the assistant or operator (whoever is zooming) to feather their start and stops together.

The Scorpio remote lens control system has a way to automate this by attaching an encoder to the dolly wheel and then regulating the focal length automatically to maintain image size once calibrated--never seen it used though.

With a remote zoom control on the gimbal of my Steadicam I've managed to hack my way through an approximation of the effect, but you really want the accuracy of the dolly to do it right.

What's the difference between this and a "trombone" shot?

What's the difference between this and a "trombone" shot?

Nothing, they are just different names for the same thing.

It's all very interesting stuff. I tried it, and it's damn difficult to do.
Is there a trick to learn it the best way? I bet the actor is the one with the best patience in all this...take 113.