Why does lens focus change with heat?

[slashmaster]

Just wondering why it is that when you set the focus on a projector just as you start the film, it changes a small amount a couple minutes later when it's warmed up? When I have my projector 10 feet from the screen and set the focus perfect while cold I have to move the screen about 6-8 inches closer to the projector when it's warmed up.

I guess some people either don't notice or don't care if it goes a slight amount out of focus. But if you do, what can you do to reduce it? Why does this happen anyway? Is it because the body of the lens expands with heat? Or because the lens elements expand?

[MovieStuff]

Just wondering why it is that when you set the focus on a projector just as you start the film, it changes a small amount a couple minutes later when it's warmed up? When I have my projector 10 feet from the screen and set the focus perfect while cold I have to move the screen about 6-8 inches closer to the projector when it's warmed up.

I guess some people either don't notice or don't care if it goes a slight amount out of focus. But if you do, what can you do to reduce it? Why does this happen anyway? Is it because the body of the lens expands with heat? Or because the lens elements expand?

It's the film that is changing the most because that is where all the heat is directed. So when the film and gate expand, it changes the back focus on the lens. You will also find that you sometimes have to refocus after transitioning from an extended dark scene to a light scene because the dark film absorbs more heat than the lighter film.

Roger

[slashmaster]

Just wondering why it is that when you set the focus on a projector just as you start the film, it changes a small amount a couple minutes later when it's warmed up? When I have my projector 10 feet from the screen and set the focus perfect while cold I have to move the screen about 6-8 inches closer to the projector when it's warmed up.

I guess some people either don't notice or don't care if it goes a slight amount out of focus. But if you do, what can you do to reduce it? Why does this happen anyway? Is it because the body of the lens expands with heat? Or because the lens elements expand?

It's the film that is changing the most because that is where all the heat is directed. So when the film and gate expand, it changes the back focus on the lens. You will also find that you sometimes have to refocus after transitioning from an extended dark scene to a light scene because the dark film absorbs more heat than the lighter film.

Roger

Ah, so you're saying the gate and the film expand and get closer to the lens and therefore change the focus? But if that were the case, wouldn't I have to move the screen farther away from the projector and not closer when that happens? Bringing the lens closer to the film is for farther away right? I certainly agree with the dark film absorbing heat part.

[Angus]

If you have a projector capable of freeze frame, you will note that the focus changes while the frame is held....and changes again when you resume motion. This is also down to the film heating up.

[slashmaster]

If you have a projector capable of freeze frame, you will note that the focus changes while the frame is held....and changes again when you resume motion. This is also down to the film heating up.

Yes! I do have a projector like that and have noticed that the focus changed while frozen. I thought it was because of that light reducing plate that moves in the way to keep too much light from burning the frame. So you're saying heat makes the film curl away from the lens and closer to the bulb and that's why this happens?