Film-Tech Forum ARCHIVE: Flat lens position and focus issue

This is topic Flat lens position and focus issue in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Frank Dubrois (Member # 3042) on 03-23-2007, 12:42 AM:

A projector of ours has a problem with its flat lens. I can't seem to focus the entire image. I can either focus the center, or the edges, but not both. This lens has performed fine until about a week ago. The focus issue came up as well as its screen position. The left side of the screen isnt as laser sharp as the other sides. I can also see the upper corner shadowed. Nothing inside the lens is loose or rattling around, the lens was cleaned, the porthole is clean. RP40 doesnt help. The trailers are pretty blurry, so we have to adjust focus for them and then adjust it back for the feature..its THAT blurry. We are using Schneider lenses. Could this possible be something with the film having uneven pressure on it going through the gate? We do use Christie projectors. I didnt get the opportunity to check out the Scope lens focus due to time contraints (it would mean running downstairs to open masking, run test loop) all with customers watching. If the Scope lens shows the same attributes, then should I be looking at the film gate? Bands?

Posted by Brad Miller (Member # 2) on 03-23-2007, 12:58 AM:

This is hard to say without being there and seeing it. Off of the top of my head.....

quote: Frank Dubrois
The left side of the screen isnt as laser sharp as the other sides.

The turret is closing too far or not far enough. There is an allen adjustment near the turret latch. (Warning this will also adjust the side to side alignment on screen.) Alternatively I don't suppose you are running soundtrack down on the platter, exaggerating the natural film curl, are you? That can exaggerate a typically not-noticeable problem.

quote: Frank Dubrois
I can also see the upper corner shadowed.

Possible the xenon is not focused properly. Also possible the turret adjustments are near their extremes and the entire thing needs be re-aligned with the lamphouse.

quote: Frank Dubrois
The trailers are pretty blurry, so we have to adjust focus for them and then adjust it back for the feature..its THAT blurry.

3K or higher xenon lamp without a heat filter causing a focus drift as everything heats up?

quote: Frank Dubrois
Could this possible be something with the film having uneven pressure on it going through the gate?

Doubtful unless your gate is seriously out of alignment.

Posted by Monte L Fullmer (Member # 2797) on 03-23-2007, 03:30 AM:

got a burnt lens, or is the elements in the lens barrel itself loose by taking the lens out of the turret and tapping on the main barrel with your finger..any rattling sounds?

amazing that a lot of turret rotations can loosen things up a bit in a lens barrel ...

..some ideas here to chek on ... - Monte

Posted by Dominic Espinosa (Member # 2122) on 03-23-2007, 12:15 PM:

If it JUST happened out of the blue it's entirely possible someone bumped the whole thing out of alignment ever so slightly.
No running in the booth!

Posted by Frank Dubrois (Member # 3042) on 03-23-2007, 12:34 PM:

quote:
I don't suppose you are running soundtrack down on the platter,

No, soundtrack up.

quote: Brad Miller
Possible the xenon is not focused properly.

Its not really a DARK shadow, like when a lamp is off center, but rather I can see the rounded corner of the aperture place.

The problem kind of looks like someone pulled the aperture plate out a little bit, and the fuzzy edge and upper corner are now visible. I checked, and the plate is seated where it should be. I can manually move the aperture a little and left side fizzyness and upper corner will look better, but then the right side has issues. Its almost as if the aperture plate shrunk and needs to be filed more. Its been fine for 6 years.

quote: Monte L Fullmer
got a burnt lens, or is the elements in the lens barrel itself loose by taking the lens out of the turret and tapping on the main barrel with your finger..any rattling sounds?

I took the lens off and looked at it, cleaned it, tapped it, but all seems fine with it. I suppose I could find the same lens on another projector and swap it to see if its something with the lens that I am not seeing.

Posted by Gordon McLeod (Member # 33) on 03-23-2007, 12:56 PM:

Well I would check the scope before doing anything else since my hunch is the gate bands are cupping the film so that it is not lieing flat in relationship to the back element

Posted by Brad Miller (Member # 2) on 03-23-2007, 01:29 PM:

quote: Frank Dubrois
Its almost as if the aperture plate shrunk and needs to be filed more. Its been fine for 6 years.

Your masking stops could have drifted as well, but I think you said you were pulling masking by hand, so that is less likely.

Gordon is right though, if you have too much tension, it can cup the film and cause that sort of problem as well. Lower your gate tension setting and try.

Posted by Frank Dubrois (Member # 3042) on 03-23-2007, 02:46 PM:

Will do.

Do gate bands wear out and cause this? I noticed that they are simply held on by a small screw on each end. if those screws come loose, could that cause the band to twist and possibly change shape of the film going through the gate? Or even worse, scratch the film? I have no clue how to tell when bands are worn out, or simply need to be adjusted.

Posted by Monte L Fullmer (Member # 2797) on 03-23-2007, 10:40 PM:

quote: Frank Dubrois
have no clue how to tell when bands are worn out, or simply need to be adjusted.

Look at the edge of each band, and the metal edge goes from thick to razor thin and sharp .. time for replacement. AND, if you are showing any cracks on the edge or even the surface, especially close to the lateral guide roller area, definitely time for replacement.

When checking for band adjustment, take the trap out, look up at the end of the trap where you can see the actual flat surface of each band and see if the bands are laying flat and square with the trapblock. If the bands seems a little off center, then you just loosen the two screws holding the band in place and center the band to flat, then retighten the two screws.

(If you got a Christie P35 unit with dbl lateral guide rollers on the trap, then you need a digeredoo device to align both bands and guide rollers before putting it back on the machine...)

Posted by Frank Dubrois (Member # 3042) on 03-24-2007, 01:16 AM:

quote: Monte L Fullmer
If you got a Christie P35 unit with dbl lateral guide rollers on the trap, then you need a digeredoo device to align both bands and guide rollers before putting it back on the machine...

Well, its a M35, not a P35, and I know the gates are different between those two models. Do I need the little thingy for the M35 as well? It looks pretty easy to loosen and adjust without a special tool, but if I need one, then I need one....

Posted by Monte L Fullmer (Member # 2797) on 03-24-2007, 03:24 AM:

Sorry, not familiar with the M35 unit - does that unit only have one lateral guide roller for the trap instead of having two of them as with the P35 series..?

Then, if you're just doing bands only, then I can say it's an easy job to do by replacing bands - just make sure that the bands are laying flat and the end strips aren't pitched slightly to either side when tightening down the screws..

(Here is the Christie part No. for the gate alignment tool:

Gate Alignment Tool, P35 SK1994-3)

good luck then .. - Monte

Posted by Darryl Spicer (Member # 711) on 03-24-2007, 11:43 AM:

I am not familiar with the M35 either but I thought I would throw this out there. On the P35GPS you have a gate pin that you pull out to open and close the gate. This pin sometimes does not seat itself into the hole causing the gate to be a little open causing focus problems. Now this would effect both scope and flat. If yours has this make sure the pin is seated or snapped into place.

Posted by Monte L Fullmer (Member # 2797) on 03-24-2007, 08:49 PM:

Gate Alignment Tool, P35 SK1994-3

Here's a couple of pix on what this tool looks like to align those P35 assemblies with:

Posted by Jeremy Weigel (Member # 4208) on 03-24-2007, 10:46 PM:

Hey Monte,

I'm using Christie P35-C's. Is this the same alignment tool for these aswell?

Posted by Monte L Fullmer (Member # 2797) on 03-24-2007, 11:03 PM:

quote: Darryl Spicer
This pin sometimes does not seat itself into the hole causing the gate to be a little open causing focus problems. Now this would effect both scope and flat. If yours has this make sure the pin is seated or snapped into place.

..I've also seen the pin seat hole,being attached to the trap by small fasteners with these fasteners coming loose causing incorrect gate closures....and the pin itself becoming difficult to close whereas the assembly has to be unscrewed out of the gate assembly and relubed so the pin has the spring movement better..

On the P35-C trap and gate ass'y, unfort, I'm not familiar with that assembly, just that if you got dbl lateral guide rollers, this is the tool to use to align them up correctly with the bands....