Film-Tech Forum ARCHIVE: Cinemascope focusing problem

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Posted by Stephen Winner (Member # 8) on 05-09-2000, 10:08 PM:

A minor issue I noticed while sitting on the orchestra level near the front watching a movie in scope the other night. The carbon-arc lamps were running off of the motor-generator, operating at 6.5KW, instead of the usual 4KW What I noticed was when the changeovers occurred, the left and right edges of the screen were out of focus for about the first 3-5 minutes. It appears after the lens is warmed up, the focus tightens up.

Anybody else running a rather large house with a long throw (150 feet) notice this issue? And is it correctable? I have a feeling it's just something we'll hafta live with due to the size of the theater,

Posted by John Walsh (Member # 168) on 05-09-2000, 10:29 PM:

What make of lenses are the anamorphics? Are they the old B&L's? How wide is the screen? It only happens at one projector?

Do you think that the projector warming up had anything to do with it- that is, could the heat expansion move the lens mount? Maybe a cooling fan is not working and the projector is getting too hot. Did the center go a little soft while the edges got better?

Regardless, I think it can be fixed, reasonably, with the correct application of money.

Try swapping lenses from one machine to the other; see what happens. There will probably be a little aperture fuzz on the sides, but this is only a one-show test. Shake the lens gently and listen if anything's loose. See if some ring or something has unscrewed. Read Brad's "tips" about focusing anamorphics.

Posted by Randy Stankey (Member # 64) on 05-10-2000, 12:29 AM:

I had that problem once, too. You could focus it perfectly but 10 minutes later it was out. Sometimes it didn't happen for days, other times it was continuous. It took a month for us to figure it out. Turned out to be a loose element inside the lens. You could hear it rattle if you took it out and shook it. The heat caused it to expand and contract so as to make it move. If it got into a "stable" position it would stay for a while, until it vibrated or expanded itself loose again.

Assuming you have the anamorphic focused correctly, that would be my bet.

Posted by John Pytlak (Member # 331) on 05-10-2000, 10:37 AM:

Stephen:

I agree that it sounds like the lens is changing as it heats up. A key question is whether it happens with only one of your lenses. John and Randy's suggestions of swapping lenses, and carefully looking for loose elements is good. Any chance there's moisture trapped inside the lens, which would vaporize and go away as the lens heats up?

Another test to run is remove the anamorphic attachment and show the image only with the "prime" lens. If there is no focus flatness problem with the prime lens, the anamorphic attachment is suspect.

With a 150-foot throw, you're probably using a pretty "long" focal length lens for scope. Whatever's happening as it warms up has to be pretty severe, as the depth of focus of a lens longer than 100mm is quite forgiving compared to a really short lens.

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com

Posted by Gordon McLeod (Member # 33) on 05-11-2000, 09:25 AM:

If it is the lens build a home made cinefocus
get a small squirrel cage blower and a length of vaume cleaner hose and bast the air stream into the space between the lens and the apperture. This will require the remocal of the should on the back of the older simplex lens mount and a hole being cut through the door.
Also it could be the gate rails heating and causing the film to cup reducing side focus
Does the problem go away if you drop the current back down to it older level

Posted by Russ Kress (Member # 461) on 05-11-2000, 11:52 PM:

I had a similar problem a while back. We had just started using 4000 watt xenon lamps.

I don't know for sure that it was the heat but i suspect that it was. We aligned the rails in the trap and the problem went away.

The trap was (is) water-cooled, but that is still allot of light and heat hitting the film.

The head is a Ballantyne Pro-35 (I know! I've heard all the jokes).

Anyway, ya might try that before spending a bazillion dollars on lenses.

Russ