What do movie theaters do about this problem?

[slashmaster]

I've got 2 projectors, an elmo 16-cl and a Bell and Howell 346 A super 8. With both of them I've got a problem where once I find the perfect focus at the beginning of the film it changes after 5 or 10 minutes as the lens warms up. The focus becomes as if the screen should be closer. If I'm noticing this with a mere 150 or 250 watts of power it makes me wonder what happens in a movie theater where some serious Xenon power is?