Film-Tech Forum ARCHIVE: What causes ghosting in the middle of the screen?

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Posted by Mark J. Marshall (Member # 1409) on 05-24-2004, 09:56 PM:

I recently noticed a lot of unsteadiness in one of our big houses, and when I looked at the tension bands, they were already set at their highest setting, so I replaced them.

For the next show I went down to the screen to watch the credits up close, and I noticed some faint sporadic smearing downward in the middle of the screen similar to shutter ghosting. I have no clue what could cause that. It's not a huge annoyance that the typical audience member will notice and complain about, but it annoys me, and I'm wondering if anyone has any hints on where to look. I'm thinking of replacing the shoes and the gate rails just to see what happens. They probably haven't been replaced in that theater in five years anyway.

I suppose it could be the print (Shrek) so I'll be interested to see if the next movie to play in there (most likely The Day After Tomorrow) has the same problem.

The projector is a Simplex 1050.

Thanks!

Posted by John Walsh (Member # 168) on 05-24-2004, 10:02 PM:

Is it only in the middle? That would be unusual. Have you tried the shutter adjusting knob? The manual is here in the manuals section.

Posted by Mark Gulbrandsen (Member # 72) on 05-24-2004, 10:08 PM:

Could be that there is a small hole burned through the center of the blade. This is not uncommon.

Mark

Posted by Mark J. Marshall (Member # 1409) on 05-24-2004, 10:12 PM:

Claude...

It can't be that because:

1) It's sporadic. The word "flutter" comes to mind. It's not a constant smear, it's a sporadic jumpy smearing.

2) It's in the middle of the screen and doesn't get worse as it goes towards the bottom or top.

Mark...
I'll check that, but wouldn't that be more constant? I will check it tho. Thanks.

By saying it was in the middle, I was basically trying to say that it wasn't "near the bottom" or "near the top". That would be the adjustment knob. It almost looks like the film is not being held steady enough in the gate for some reason and every once in a while, it flutters or jumps around a little bit. Well, actually I guess it jumps UP since it's smearing downward. Whether that's the intermittent sprocket or something else, I'm not sure.

Thanks!
Mark

Posted by Joe Redifer (Member # 3) on 05-24-2004, 10:22 PM:

Why is Claude referred to in this thread?[av=d]

Posted by Randy Stankey (Member # 64) on 05-24-2004, 10:37 PM:

Have you considered that the light from the projector may be going through the perforations in the screen, hitting something behind the screen then reflecting back to the viewer's eye? (Through another perforation.)

I see this all the time. My theater is also a legitimate stage. When the screen is flown in a full-black traveller (a full-stage width velour curtain that opens and closes) MUST be flown in behind the screen or else the viewer WILL see reflections off the upstage (back) wall.

Even if the object(s) behind the screen are painted black you can STILL see this effect because even a black surface can reflect. This is especially true during credits where there are white letters on black background.

My black traveller is especially made to combat this reflection. It is a black velour that is made so that it doesn't reflect (very much) light. Plus, it is pleated so as to provide a non-uniform surface which will mimimize reflections even more.

The way to tell is to move around in the auditorium while looking for the effect. Duck down... Stand on tip-toe... Walk back and forth along the rows... Move front to back in the auditorium. You will see the "ghost" image move in relation to the "real" image.

If true, then go behind the screen while the picture is playing. Look around for a "ghost" image being projected onto walls, speakers, platforms and other objects benind the screen.

To combat this, drape black fabric on the offending object(s). Crumple it up to make an uneven surface. Try moving objects behind the screen. Just rotating the object a few degrees may do the trick. Maybe all you need is a can of flat black spray paint.

One caveat to draping fabric: It must be fire retardant. Black cloth you buy at the fabric store or Wal-Mart is usually not fire retardant.

Check out BMI Supply.

Black Commando Cloth with fire retardant is only $5.00 per yard.

Black velour with "Inherrent Fire Retardancy" is only $12.00 per yard.

Posted by Phil Hill (Member # 371) on 05-24-2004, 10:38 PM:

Joe! You're cracking me up here, bud! Hahahahhaha

>>> Phil

Posted by Paul G. Thompson (Member # 655) on 05-24-2004, 11:07 PM:

Is it a smearing flutter or a focus flutter or both?

How steady is the image on the screen? Are there any hotspots? Everything running nice and cool temperature-wise? And, did it just start doing this, or is it an ongoing problem that might have not been noticed in the past?

I have seen this happen in some XL's that were ready for a complete overhaul.

Posted by Mark J. Marshall (Member # 1409) on 05-24-2004, 11:46 PM:

quote: Joe Redifer
Why is Claude referred to in this thread?

Cause I guess I'm kinda tired, and typed Claude instead of John.

Sorry for the confusion! [Roll Eyes]

Posted by Kenneth Wuepper (Member # 1174) on 05-25-2004, 07:02 AM:

Mark J.
You didn't say what machine this is. If, in fact, it has a drum shutter it could easily be a timing problem as the critical closing and opening of the light path is in the center of the image. Bad timing can also cause the image to appear to jump on the screen and this would not be correctable with gate tension adjustments.

KEN

Posted by Demetris Thoupis (Member # 833) on 05-25-2004, 07:06 AM:

The might not be ghosting. Maybe your lamp is too old and the light is flickering. Try checking the age and that you are running it on the suggested current and not above or lower.
D

Posted by Mark J. Marshall (Member # 1409) on 05-25-2004, 08:52 AM:

Thanks for the replies so far, everyone.

Randy, It's a torus screen, so no perfs.

Paul, I'm going to go back in tonight and look at the things you're asking.

Ken,

quote: Mark J. Marshall
The projector is a Simplex 1050.

Demetris, I'll check the lamp hours tonite too.

Posted by Darryl Spicer (Member # 711) on 05-25-2004, 09:10 AM:

Run some unused trailers through the machine. It is possable that the film may have been slightly damaged by heat.

Another possability is that once you changed the bands the gate may not be closing in far enough. I have at times had to readjust the stop point of the gate after changing very old bands.

Posted by Peter Hall (Member # 695) on 05-25-2004, 09:42 AM:

Hi

I can't see how a hole in the shutter would cause a problem only in the middle - surely a blade shutter with a hole in it would give a smear right down the picture ??

Also if a drum shutter - mistiming would cause a problem predominantly at the top or bottom, with a little leakage on the opposite side but not in the middle.. (am happy to be corrected however)

Question - is the effect more prominant on a black scene with a little white, eg white on black credits.. ? If so, or in any case I'd suggest a heat problem caused by maladjustment of the lamp. Try removing the lens, then making sure the bullseye is central. If so back off the lamp to de focus it and make the light duller but more even - does this fix it ? If so look at lamp focus. Or lenses - something as silly as a fingerprint on the lens ? Does it do it on both ratios, if so which is worse ? Would be interested to know the outcome..

Posted by John Pytlak (Member # 331) on 05-25-2004, 11:59 AM:

It's just these kinds of situations that a good test film like the SMPTE 35-PA (RP40) or Schneider test film are made for evaluating.

http://www.schneideroptics.com/projection/cinema_projection/test_film/

http://www.smpte.org/smpte_store/test_materials/

The SMPTE "Universal Jitter, Weave, and Travel Ghost" film is also useful.

Posted by Ken McFall (Member # 845) on 05-25-2004, 05:23 PM:

Peter I have to disagree with you.... a drum shutter by its nature has two effective leading edges, one falling and one rising. A normal flat blade shutter only has one leading edge. Therefore travel ghost would be most noticable in the middle of the frame as the middle is blanked out last and exposed first!

Posted by Matthew Jaro (Member # 1786) on 05-25-2004, 08:52 PM:

Is your port glass parallel to the screen? It could be reflections from the port. You would probably only notice this with white titles on a black background. Smeering sounds more like a reflection rather than the jitter that would be caused by film instability.