Film-Tech Forum ARCHIVE: IMAX GT Right Eye Focus Puzzler

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Topic: IMAX GT Right Eye Focus Puzzler

Brian Michael Weidemann
Expert cat molester

Posts: 944
From: Costa Mesa, CA United States
Registered: Feb 2004

posted 03-17-2004 10:01 PM

Yesterday, all the morning shows of NASCAR 3D ran perfectly. Then, at the start of the 4:00pm show, the Right Eye was really out of focus, while the Left Eye was perfect. Using the focus pendant, I had to pull the main focus all the way down to 000 just to get the Right Eye passable, but not perfect.

I'm 99% sure nothing was bumped during thread-up and, anyway, all lenses were secure when I checked everything after the show let out. The main lenses were still flush with each other, as they had been at our service, which was last week.

I ran test loops this morning and had to manually pull the Right Eye main lens back about a millimeter (and then fine tune with the focus pendant) to get things perfect. But, since our Trim Focus adjusts the position of our Left Eye lens mount, both my tech and I are baffled as to how it happened in the first place.

Any ideas?

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Adam Martin
I'm not even gonna point out the irony.

Posts: 3686
From: Dallas, TX
Registered: Nov 2000

posted 03-17-2004 11:20 PM

That's an interesting one. Let's eliminate the impossible:

It's probably not electronic. In order for the right eye to be out of focus (by normal electronic means), the entire lensmount needs to move, including the left eye. That would make the left eye just as out of focus as the right. In this case, for the left eye to be back in focus, the trim focus would have to be adjusted as well, and the exact amount that the main focus was pushed out. Not likely.

That having been said, go in the auditorium and check your remote plate. Make sure it is intact and the cover has not been knocked loose and reassembled by some well-meaning person who may have knocked it with a broom and pinched a wire or two putting it back together. (Not that I've had that happen or anything. [Wink]

) Now is as good a time as any.

If the main lenses are indeed fairly close to even with each other (mine are about 1 mm from each other) and the positioning rings are properly attached, I would look at a couple of things in the lensmount. The most important is whether or not the film is actually running through the aperture block properly. Check clearances, registration pins, pressure bars, wiper bars, etc. Also verify that the lensmount feels "normal" when closing and something isn't binding or rubbing somewhere it's not supposed to.

Check the field lens. Is it seating properly and operating normally? You should not be able to move it toward/away from the film plane much, if at all. The tensioning bands in the lens mount should pretty much hold it in the same plane as the film and rotor. Is the positioning piston operating properly and not loose or bent?

Check the spherical lens assembly. It could be that it was not reinstalled properly during service and has vibrated loose or has finally fallen into place. Did you have to adjust lens focus after putting the front end back together during service?

As far as anything mechanical with the lens mount, there are several versions of the GT lens mount. Mostly, it comes down to the right lens only being adjustable for lens shift. So, unless the lens itself moved in the mount, there's not much to go wrong there.

There's a start.

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Brian Michael Weidemann
Expert cat molester

Posts: 944
From: Costa Mesa, CA United States
Registered: Feb 2004

posted 05-27-2004 04:21 PM

A few days ago, this exact problem recurred, however it was in opposite direction the right lens had to be moved ... a clue!

After the show with horrid presentation, I adjusted the right main lens, pulling it out about a millimeter. As I was tightening the screws back, something shifted! Since the left eye is the trim focus one, it's mounted separately to the main block. The right eye lens is mounted to that rectangular post on its right, which itself is fastened to the base of the block with four screws ... not all of which were tight! Basically, the whole right lens mount would pivot, with a range of about a millimeter forward, if given enough force. And it could pivot back.

I moved it to its original position, tightened the screws, verified it could not shift again, and went on screen with the next movie, did some fine tuning on the focus, and problem solved!

It must have been an oversight, but the lenses couldn't have been flush with each other both before and after the shift, when I noticed the problem the first time.

Looks like someone tends to hang on that post when threading.

Thanks for the suggestions, in any case, Adam.

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