Film-Tech Forum ARCHIVE: Focusing trouble.

This is topic Focusing trouble. in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by David Whitbeck (Member # 894) on 02-20-2002, 04:22 PM:

My problem here is that whenever running a flat print, I can get it crisply focused in the middle but then as you look farther and farther towards the sides, it gets more and more out of focus. So I adjust the focus so the sides are crisp but then the middle is quite out of focus. I've compromised between the two, but I'm not getting near the quality that I could be. The trap and the gate are kept clean so there is no build up in that area. Could it be the gate tension? It's a Simplex 35 and the gate tension is on the 3rd notch due to the continuing deterioration of the the intermittent and the fact that the gate starts making noises and the pads start moving a lot if I turn it down any. Maybe somebody put the lense together wrong? I don't know. Thanks for any help you can offer.

Posted by Paul G. Thompson (Member # 655) on 02-20-2002, 04:39 PM:

Check your xenon lamp alignment. The lamp may be out of focus. Also, check to see if the mirror coating has not flaked off, or the dichloric coating on the filters are not flaking away if you are using them.

Also, inspect your film trap to make sure the pressure pads are clean, as well as the rails. Check the gate alignment, as well as the film lateral guide. It could be pinching your film excessively.

Hope this helps....

Posted by Jonas Heijkenskjold (Member # 1085) on 02-20-2002, 04:50 PM:

Couldn´t that be a heat problem? Are you using hot or cold light mirror? Heat filter in lamp house? If the exposed film gets hot it can curve in the middle and get out of focus.

Posted by David Whitbeck (Member # 894) on 02-20-2002, 05:02 PM:

But if any of these things were the problem, wouldn't I have the same focusing problems with scope movies? Although, our screen is a flat ratio so I may not be able to see that the scope is having the same problem. Still, I haven't seen anything even close with the scope.

Posted by Jerry Chase (Member # 660) on 02-20-2002, 05:03 PM:

Flat screen or curved? Angle of projection? Lens mfg.? Focal length? f number? Does it happen on just one print or on any print?

Paul has the good suggestions on what to check first.

Posted by Darryl Spicer (Member # 711) on 02-20-2002, 05:47 PM:

Check your lens elements. Make sure they are not loose. Sometimes a loose lens element, if loose enough will cause a focus problem. A note on that tension knob for the preasure straps. You may want to get new straps and replace them. It is better to run that preasure down near 1 or 2 and still obtain a steady picture. Running 3 is ok but anything above that is a sign that the straps are wearing thin and could snap at some point during a show.

Posted by Darren Briggs (Member # 1071) on 02-20-2002, 05:52 PM:

Have you tried another lens from another screen??

I had the same sort of problem at a cinema I service on there Scope picture. It was the scope backing lens which caused the problem.It was ok for the first 3 mins then went right out, By shutting the lamphouse douser a touch and limiting the light it focused fine.
A new les cured the problem.

Darren

Posted by Brad Miller (Member # 2) on 02-20-2002, 05:57 PM:

This really sounds to me like a xenon that has been focused down too tightly and there is excess heat in the center of the film. Do you notice the focusing gets worse during end credits where there is white text over a black background? If so, then this is definitely heat related.

The reason you are not seeing it so much on scope movies is because flat movies require more of a blowup, due to the smaller surface area of film in the aperture. If this screen has common width masking like most stadium seating theaters have, then this is greatly exaggerated.

As always, we need to know more than the info you have given us. Otherwise we're all just taking stabs at the situation without really knowing the specifics. Post again and this time make sure you tell us:
Make and model of lamphouse
Wattage of xenon bulb in lamphouse
Is there a heat filter between the lamphouse and the projector?
What kind of lenses are you using?
Is this a common height masking setup or a common width masking setup?
Curved screen or flat screen?
Screen brand and surface type (if you know).

That'll help us.

Posted by David Whitbeck (Member # 894) on 02-20-2002, 06:22 PM:

The lamp house is a Cinema Film Systems SX2000 with a 2000 watt bulb. The lense is a Super Sankor 2.5 inch F 1.9. The screen is flat and has no masking, it's ratio is about 1.85:1. There is no filter in between the lamphouse and projector. I can't tell you anything more about the screen. Sorry for the lack of info, I was at school when I posted.

Posted by Paul G. Thompson (Member # 655) on 02-20-2002, 06:59 PM:

By the way, did this problem happen all at once over the last several days? Or, has it been a nagging problem from day one?

In addition to what Brad mentioned, the primary lense being used with the scope attachment might be a slower lense. That gives a better focus field. What is the f-stop in the scope primary?

I think the xenon is out of alignment. By the way, what is your Xenon current? Are you over-driving the bulb and it is getting too hot? Also check your stack blower. Without proper xenon venting, things are going to get hotter than hell. If your aperature plate it "too hot to handle", then you definately have a lamphouse associated problem in one area or another.

Posted by Jerry Chase (Member # 660) on 02-20-2002, 10:08 PM:

IF what you said about being able to get the center into crisp focus is an accurate description, and IF there is no characteristic buckling of the print from overheating, then my bet is that something is wrong with the lens.

If there were overheating occurring because of your reflector being mis-aligned, the film would "pop" in the gate and part of the time the center would be in focus, and part of the time it would be out of focus. That could make getting a crisp center difficult (not impossible, but difficult unless the deformation occurred very late in the open shutter cycle).

There should be a pic around here somewhere of a film with heat damage. I'm guessing you would notice it right off if you were having focus problems.

The flat screen rules out curvature problems.

With this guess and a buck, you might get a cheap cup of coffee.
Can you get a loaner or replacement lens in for comparison?

Posted by Stan Gunn (Member # 554) on 02-21-2002, 01:00 AM:

It appears to be a lense problem called spherical abberishion (sorry about the spelling) this is found a lot with cheap16mm lenses,but I have seen it with a lot of 35mm lenses also.
The only way to fix it is a new lense, given the focal lengh of the lense I assume that you have a short throw and a large screen size.
With the lense type you have, I am certain a better lense will cure your problem.

------------------
KALEE FOREVER. CARBON ARC, THE ONLY LIGHT FOR THE STARS!
ALL PARTS FOR VICTOR AND KALART VICTOR 16MM PROJECTORS.SERVICE TO 35 AND 16MM PROJECTION EQUIPMENT.
35MM sprockets made to order.

Posted by John Pytlak (Member # 331) on 02-21-2002, 08:30 AM:

Here are my articles on heat-related problems:
http://www.kodak.com/US/en/motion/newsletters/reel/fall97/pytlak.shtml
http://www.kodak.com/US/en/motion/newsletters/reel/winter97/projection.shtml

In general, use the lowest gate tension possible that still gives a steady image.

Very short focal length lenses have much shallower depth of focus.

Both Isco and Schneider have done much work in the last 20 years to optimize lens designs to achieve uniform focus across the screen. This work has been recogized by the Academy of Motion Picture Arts and Sciences with multiple awards.

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
Rochester, New York, 14650-1922 USA
Tel: 585-477-5325 Cell: 716-781-4036 Fax: 585-722-7243
E-Mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion

Posted by Gordon McLeod (Member # 33) on 02-21-2002, 08:48 AM:

Number one problem is probably the Sankor lens

Posted by Pete Naples (Member # 761) on 02-21-2002, 12:46 PM:

I'm with the lens too! If the trap was out of kilter and buckling the film you'd see it on 'scope too. I've replaced loads of those old Sankor lenses, and the customer comment is always that the picture is loads sharper, brighter, and is a few cases that it's lost the green tint it's had for years!

Posted by Paul G. Thompson (Member # 655) on 02-21-2002, 01:20 PM:

Sure hope David gets back with the info we asked. If his problem was an on-going thing for a long time, I would bet it is the Sankor Lense. He should also check the lense to see if it is burned, separating, has loose elements or has been dropped.

I think the key is how long has this been going on. If he has no problem with a scope primary having the same f-stop as the flat lense, then I would say he definately has a problem with the flat lense, and a replacement for that may be indeed needed.

In the lense area, my knowledge is anal. But it is just a thought.

Posted by Dave Williams (Member # 299) on 02-21-2002, 03:07 PM:

I have had this same problem in the past. What I found was that the xenon bulb was focused too tight and burned the flat lense in the center. As a result, the image on screen was soft and yellow in the center, and impossible to focus throughout the image. We had the lense resurfaced and it was just great after that.

The reason the effect was not noticed on the scope lense, was because the scope lense was manufactured using different grade optics and did not burn.

Dave

Posted by David Whitbeck (Member # 894) on 02-21-2002, 03:19 PM:

The problem I believe has been there for a long time. Some movies it's more noticable in than others. Paul, you're talking about a scope primary? Is that the the flat lense that is attached to the anamorphic? If that's the case, we have a Schneider 4.13 inch F 2 attached to a Cine Navitar Animorphic. The aperture plate never feels like it's overly hot, I pull it all the time and it's decently warm, but I don't think it would burn me, I'll find out after this next show. The Sankor looks OK, but my knowledge is less than... But it looks OK. Seems to be tight all around, too.

Posted by Joe Beres (Member # 678) on 02-21-2002, 03:25 PM:

It sounds to me like it could be the lens. Is it more noticeable is darker films, scenes, and shots, and less noticeable in brighter ones? Brighter shots will allow more light to be projected, sometimes allowing more contrast and make focus troubles more apparent.

This post was edited, because due to a lack of clarity and my own scattered mind, it made very little sense.

Posted by David Whitbeck (Member # 894) on 02-21-2002, 03:30 PM:

The best time to see the problem is when there are words on the screen such as the credits.

Posted by Gordon McLeod (Member # 33) on 02-21-2002, 03:35 PM:

Sankor lens are not the most durable or high quality until very recently they were still cemented elements

Posted by David Whitbeck (Member # 894) on 02-21-2002, 04:16 PM:

I have lowered the gate tension to 1 with no change. If it is just a cruddy lense, I would have to say that it will make me happy because then I'm not to blame

. We're about to be getting new screens and installing some Christie P35C's so hopefully we'll acquire some new lenses at the same time.

Posted by Pat Moore (Member # 380) on 02-21-2002, 08:05 PM:

I'd have to vote on the lens being the bad guy as well. Those were not the greatest lenses ever made, problems with coatings as well as the glass. No depth of focus at all with them.

Posted by David Whitbeck (Member # 894) on 02-21-2002, 10:04 PM:

Well guys, if the lense is at fault then thanks for all your help. I'm going to try to run some test loops through tomorrow with our other flat lense, I'll let you know the results I guess. We only have two screens here so it's kinda tough switching the lenses when they are both in use. Thanks again.

Posted by Stan Gunn (Member # 554) on 02-21-2002, 11:29 PM:

I did not want to be that blunt.

Posted by Paul G. Thompson (Member # 655) on 02-22-2002, 01:24 AM:

David W.
If you have the f-2 lense as the scope backup, you will have a good focus field. You said the aperature felt ok temperature wise, so I don't think you have any serious mis-alignment problems.

I now change my view and agree many of the forum members. I think it is time to get another flat lense. But remember one thing.....The faster the lense is, less focus depth you have to play with. However, the faster lense will pass more light. Conversly, the slower the lense is (higher f-stop) the better the focus depth, but the light loss will be greater. It is all a trade-off. If you have a high power xenon and have light to "burn", you would probably be better off with a slower lense.

As far as the flat lense is concerned, (the Sankor) it is more than likely the culprit giving you all your grief.

Hey guys, correct me if I am wrong.

Posted by David Whitbeck (Member # 894) on 02-22-2002, 03:53 PM:

Just so everyone knows, I swapped our two flat lenses and ran a loop through and sure enough, the other flat focused fine. So, I guess we have a shitty lense, you wouldn't be surprised if you saw our theatre... Anyway, thanks for all your help. Too bad it wasn't something easilly fixable. Take it easy.

Posted by Joe Beres (Member # 678) on 02-22-2002, 04:57 PM:

Well, obviously you will need to replace that lens. If you're strapped for cash, you should be able to find a good-quality, used lens at a good price. We have all Isco lenses here, some of which were purchased used, and they have not given us any trouble.

Posted by Jerry Chase (Member # 660) on 02-22-2002, 05:04 PM:

I didn't say this, but lenses can be accidently dropped...

Posted by David Whitbeck (Member # 894) on 02-23-2002, 06:06 PM:

I'm not going to be the one to drop it... We'd be screwed if we lost that lense, we're running to flat movies right now that both play at the same time so you get the picture, but I suppose if I really got tired of it, it would be an option

. Well, we're supposedly getting new stuff in April, so I think we can hold out a little longer, I was just hoping it could be fixed.

Posted by Stan Gunn (Member # 554) on 02-23-2002, 09:51 PM:

You could curve the screen to match the lense.

Posted by Gordon McLeod (Member # 33) on 02-23-2002, 10:06 PM:

Best cure for a old Sankor lens
Place on cement block
Hit with sledge hammer

Posted by Paul G. Thompson (Member # 655) on 02-24-2002, 01:12 AM:

Gordon, I found they make a wonderful magnifying glass for reading the medication instructions on medicine bottles...

Posted by Stan Gunn (Member # 554) on 02-24-2002, 08:16 PM:

A paper weight would be another good use.

Posted by David Whitbeck (Member # 894) on 02-24-2002, 08:26 PM:

You guys wouldn't happen to be laughing at me and my situation would you...

Posted by Paul G. Thompson (Member # 655) on 02-24-2002, 10:04 PM:

David, that is for us to know, and you to guess...

Posted by Chris Markiewicz (Member # 1415) on 12-18-2002, 07:09 PM:

I also have a burned flat lense - Kollmorgen Super Snaplite BX241 2-1/2 inch - with a nice soft brown image. The only other lense available is a Super Sankor which like David's won't focus the entire field. I'm researching a possible replacement. Any opinions on Cine-Navitar? Thanks in advance.