Film-Tech Forum ARCHIVE: How long can I run a loop without damaging the lens?

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Posted by Frank Dubrois (Member # 3042) on 07-10-2006, 09:19 PM:

Like the title says, If I need to adjust a lens for whatever reason, how long is it possible to run a loop of film before the les could crack? 3k and 4k bulbs.

Posted by Peter Mork (Member # 1336) on 07-10-2006, 09:58 PM:

I don't think running a loop of film is any different from running a print, insofar as the idea is you wanna shield the lens from the heat you'd get from having nothing at all in the gate. So as long as the loop doesn't break, you're safe.

I have tried experimenting with a junk lens and no film - if you hear a "dink" sound, that is your lens fracturing.

Posted by Frank Dubrois (Member # 3042) on 07-10-2006, 10:55 PM:

Well I know a loop of film heats up a lot more than if I were running a print. I was just curious if I ran the same loop nonstop for like 10 minutes, could that damage the lens..I really didnt wanna find out by expermentation because I know the lenses are quite expensive.

Posted by Mike Blakesley (Member # 26) on 07-10-2006, 11:36 PM:

Each frame of the film will cool as it exits the projector. If you have a loop made with say 10 feet of film, I doubt the temp of the film would rise much at all -- certainly not enough to damage a lens.

Posted by Monte L Fullmer (Member # 2797) on 07-10-2006, 11:49 PM:

When I ran Christie SLC-45 consoles with 6k bulbs in them, I made a 20 ft loop that would run completely through the machine and to the top roller of the platter...like a pyramid threading path. Film had time to cool and it kept some of the heat off of the lens when I was doing 35PA (RP-40) screen loops. But, I made sure that I was done within a very certain time limit do to the heat that the elements were soaking up-but not as if naked white light was passing though them...in which they will shatter in a heartbeat with that much wattage of between 4 to 6k bulbs..(saw one shatter to bit due to an accidental strike and the dowser was open..but the install tech did that one....busted a scope element to bits...)

If one wants to really keep heat down, do a bunch of credit end loops from trailers - being black with white lettering...but still do the 20ft plus loop length... to let the film cool down before re-entering the machine.

Then, with this loop lens will be not as critical to breakage and the loop can run for quite the time.

-Monte

Posted by Frank Angel (Member # 248) on 07-11-2006, 03:44 AM:

I don't quite understand the relationship between the length of the loop and protection for the lens. True, with a short loop, the film will get quite hot as it will not cool down before it hits the aperture again and again, perhaps it might even sustain physical damage like warping. But if the film remains between the lens and the light source, then it's absorbing the heat the same as if a reel of film were running thru the machine. Shouldn't that be able to run on indefinately or until the film physically breaks down due to overheating? Or am I missing something?

Posted by Peter Mork (Member # 1336) on 07-11-2006, 05:33 AM:

I'm with Frank; I can't see that it matters if the film heats up or not - it's still giving the same shielding effect.

Presumably dark film will absorb more heat than clear film. Here's the corollary question: if some evil filmmaker decided to make a film that was all white and no little or no image, would lenses across the nation dink out? (How much energy does the base alone absorb?)

Posted by Brian Hogan (Member # 940) on 07-11-2006, 05:43 AM:

i thought i read someplace that even running just the white light (shutter spinning, of course) through the lens was ok for up to about five minutes. i agree as well that film in the gate is film in the gate, no matter how long the loop is.

i have been witness to a lamp focusing incident in which the operator was focusing the lamp without the motor running. did not take long at all for the scope lens to crack. prolly less than a minute for a 6K lamp.

Posted by John Pytlak (Member # 331) on 07-11-2006, 09:18 AM:

Running a loop of film would reduce the radiant energy level just like running a full length film would.

Running "open gate" (no film) definitely can damage a lens -- how fast depends upon how large the lamp is and the efficiency of any heat filtration (dichroic mirrors). Any misalignment of the lamp could also cause additional lens heating.

Posted by Paul Mayer (Member # 355) on 07-11-2006, 10:46 AM:

Potential lens damage also depends on the lens' construction. Older lenses used construction techniques like cemented doublets and the cement (Canada balsam) could easily scorch when subjected to too much heat. One had to be really careful with the length of heat exposure when using these older designs.

Newer lens designs don't use cemented techniques and can therefore take longer exposures and more heat, but the glass elements can still shatter if subjected to high enough heat loads.

Posted by Randy Stankey (Member # 64) on 07-11-2006, 11:30 AM:

I agree that, as long as there is film in the gate you'll be OK.

As long as the film isn't getting so hot that it risks burning through and/or breaking there shouldn't be much concern. However, I would still keep an eye on the film to see that it isn't getting too hot. If it's still warm as it re-enters the projector, just make the loop bigger until you are comfortable with it. 10 ft. ought to be enough with PA-35/RP-40. It wouldn't hurt to use more if you can afford it. (Larger loops are generally better anyway.)

If you are concerned about the heat you could carefully reach in and feel the lens and the inside of the projector around the lens mount while it is running a normal show and guage how hot it feels compared to when you are running a test loop. (Or even better, use a non-contact infrared digital thermometer.)

If the projector and lens are getting significantly hotter during testing and alighment than they do during a real show, stop working and shut the dowser or shut down the lamp for a while until it cools. Resume work when you are comfortable with the temperature.

But as long as you have good film in the gate and all other things are in working order I wouldn't worry about it too much.

On a scale of 1-5, ("1" being "No worry at all." and "5" being "Oh Shit! It's going to break!") I'd give this a "3"... About as much concern as you give anything when you work.)

Posted by Louis Bornwasser (Member # 3063) on 07-11-2006, 12:00 PM:

We did considerable testing (due to burnt film) with a loop of green rating strip deliberately made as small as possible. (Century JJ and Super 80 running 150 amps on a 4 kw bulb)

The lens being damaged is no factor. When the bulb is focussed correctly, there is no film damage either, even after 90 minutes. Loop was 9 ft. Louis

Posted by Frank Dubrois (Member # 3042) on 07-11-2006, 03:14 PM:

OK sounds like if I make a 10ft loop and occasionally monitor the lens, all should be fine. Thanks!