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Author Topic: Achieving uniform focus Ken Lackner Phenomenal Film Handler Posts: 1907 From: Atlanta, GA, USA Registered: Sep 2001 posted 05-09-2004 01:02 AM                     If your projector is tilted at an angle towards the screen, is it ever going to be possible to acieve a perfect focus top to bottom? I notice a lot of times I can get the bottom in focus but as I look closer to the top the picture becomes less focused. I assume this is due to the projector being titled. Am I just going to have to accept this as a fact of life? What about curved screens? I would think that it would be hard to acheive the same focus all the way across. Am I right? (I haven't looked, I'm just asking about that one because I'm curious how it works.)  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 05-09-2004 07:53 AM                     Ken, We need more information...projector type, lenses, screen size, throw, tilt angle, etc., in order to help you. Mark  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 05-09-2004 01:47 PM                     It will usually entailing shimming the lens mount and irising down the lens if it is very steep  |  IP: Logged Scott Norwood Film God Posts: 8146 From: Boston, MA. USA (1774.21 miles northeast of Dallas) Registered: Jun 99 posted 05-09-2004 10:02 PM                        The keystone/focus issue can be fixed in a couple of ways. First, the screen frame can be angled to match the projector tilt angle. This results in a properly focused image without keystone distortion when viewed from the booth. Another option would be something like the Schneider "Cinelux PC Keystone Correction Adapter" which I recently found out about. It's the product that I have been wanting for years. The concept is the same as that behind the use of movements with a view camera to eliminate perspective distortion. The projector would be mounted such that the film runs parallel to the screen (no tilt) and then the lens itself would be shifted to position the image correctly on the screen. I assume that this is useless for extreme tilt angles, but might work nicely when limited correction is required. Hopefully someone who has used these can give more detail on their effectiveness.  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 05-09-2004 10:08 PM                     Ken, Again more information would be helpful as there are adjustments in some projectors that if set wrong could cause your problem. You need to know if its a problem caused by excessive down tilt or by a mis-adjusted projector, or a worn part..... Scott Norwood said.... "Another option would be something like the Schneider "Cinelux PC Keystone Adapter" which I recently found out about. It's the product that I have been wanting for years." _________________________________________________________________ Well, its been available for at least 15 years and probably longer. ISCO also makes a PC corrector thats about half the price of the Schneider. I recently installed one of the new black Schneider PC correctors and its a piece of crap. The little adjuster to make the inner barrel shift back and forth is not worth anything and I ended up taking this part out of mine. And when you loosen the clamp screws to allow adjustment the inner barrel then sags down from the weight of the lens making fine adjustment really fustrating. The better of the two is definately the ISCO and oddly enough Schnelder used to make one exactly like it...why they went to the new design I'll never know. Mark  |  IP: Logged Richard Fowler Film God Posts: 2392 From: Ft. Lauderdale, FL, USA Registered: Jun 2001 posted 05-09-2004 10:17 PM                     I used the older Schneider PC correctors in a screening room which had a building support in the middle of the booth wall and the machines where 12 feet apart for a 55 foot throw...no problems. the only problem would be two - three service calls a year because some new operator would shift the projectors since they where "pointing" in the wrong direction  |  IP: Logged Adam Martin I'm not even gonna point out the irony. Posts: 3686 From: Dallas, TX Registered: Nov 2000 posted 05-09-2004 10:31 PM                     A misaligned lamphouse can also cause focus problems. With modern lenses, screen tilt/curve and projection angle should have little effect on focus as long as the lens is properly aligned with the film plane.  |  IP: Logged Carl Martin Phenomenal Film Handler Posts: 1424 From: Oakland, CA, USA Registered: Feb 2002 posted 05-10-2004 03:24 AM                        quote: Scott Norwood The projector would be mounted such that the film runs parallel to the screen (no tilt) and then the lens itself would be shifted to position the image correctly on the screen. could one not achieve the same result by cutting the aperture high in the plate and perhaps adjusting the position of the lamp? carl  |  IP: Logged Michael Schaffer "Where is the Boardwalk Hotel?" Posts: 4143 From: Boston, MA Registered: Apr 2002 posted 05-10-2004 04:45 AM                        quote: Scott Norwood First, the screen frame can be angled to match the projector tilt angle. Then you are sitting in an auditorium watching the tilted screen. In my experience, this is far worse than having a little keystoning on the screen since your eye adapts to that while the tilted screen is simply an odd element in a room where you have other visual references.  |  IP: Logged Matt Zeiner Expert Film Handler Posts: 146 From: Windsor, CT USA Registered: Sep 2003 posted 05-10-2004 06:01 PM                     I'm with you, Michael. If there is keystoning, there is keystoning. Thats all. I have fiddled around with a few of those correction gizmos - they kinda work if the keystoning is not too severe, in which case, why bother with it at all. The only time I could see justifying keystone correction is for 16mm blowups. A well-cut aperture will mask minor keystoning. If the keystoning is unacceptable, the building is unacceptable. Perfect is nice but generally unattainable with band-aids.  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 05-10-2004 06:39 PM                     The maximum correction with a PC adaptor is about as though you are 7 degrees off dead center. The other good use is for perfectly aligning all the lenses for different formats in a single barrel machine. Mark  |  IP: Logged Matt Zeiner Expert Film Handler Posts: 146 From: Windsor, CT USA Registered: Sep 2003 posted 05-12-2004 06:14 AM                     quote: Mark Gulbrandsen The other good use is for perfectly aligning all the lenses for different formats in a single barrel machine. Good point though I have yet to run across a movie house ( or screeening room) that could afford that kind of tweakeage who wouldn't opt for a turret in the first place. Matt  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 05-12-2004 08:39 AM                     Matt, You don't run across that many screening rooms...obviously. I have literally 100s of PC adapters out there. The ONLY place I've installed with turrets are for main-stream cinema. Most the screening rooms I deal with have 4-5 lenses per machine (1.85, 1.66, 1.37, Scope, Silent and then there may be one or two lenses for 16mm). Turrets don't exactly work there. We just installed PC adapters in Ambler, PA to deal with some extreme keystone issues as well. The PC adapter gives the experienced tech a lot of control over the picture placement and appearance. ISCO's will shift as much as 5mm in any direction...Schneider's new one will shift a lot more but it is more pricey. However, when dealing with Silent/super-35 or with long focal length lenses they are quite valuable tools. However, with that being said...in the typical Flat/Scope theatre, they are rarely necessary. Steve  |  IP: Logged Frank Angel Film God Posts: 5305 From: Brooklyn NY USA Registered: Dec 1999 posted 05-12-2004 08:48 AM                        In the old Brandt theatres on Forty-Douch Street in Manhattan, most of them, like the Victory, the Amsterdam, etc. had booths sitting up 5 and 6 stories above the stage and with very short throws. The angle was insane. All these theatres had fly-hung screen frames on the first or second pipe of the stage (they originally all did vaudeville stage shows along with films, and hence they were designed very shallow so actors' voices would carry to the back of the theatre). The solution all those theatres used to deal with severe keystoning was to simply grab the bottom of the screen and pull the sucker downstage as much as needed to get it fairly close to being parallel to the projector lens. The keystoning would have been terrible without doing that. It didn't correct keystone distortion completely (the damn screen would have had to be laying practically flat on the stage for that), but it was reduced enough to become livable. And that tilt in the screen, from a young audience member like me, looked kinda cool. Sort of reminiscent of the CinemaScope curve, which it turns out, some of those old houses still maintained WITH the tilt. In my theatre, the booth is 5 stories up, but the throw is much longer (I think about 120ft) so that minimizes the keystoning error, but we still needed to get a hold of special brackets for the take-up magazines which thrust the magazine forward so it does not hit the pedestal. Without it, you can't get the angle you need without crushing the magazine against the pedestal base as you crank the tilt adjustment wheel. I also had to get a roller from Kelmar to push the take-up belt so it didn't rub against the Simplex soundhead casing. Interstingly, we tilt the screen DOWN toward the middle of the orchestra, not up toward the booth (which, in fact, INCREASES the keystone error). We do this simply because we need as much light as possible to be reflected back to the center of the orchestra, NOT back up to the top of the balcony. We can live with the keystoning which is properly masked away by the aperture plate and the masking, but we can't loose all that light to the heavens. Living with some keystone distortion in the image geometry (but still maintaining a perfectly rectangular looking screen) is the much lesser of two evils. Of course none of this screen tilting chicanery will work on anything but matte screens surfaces. Any gain screen will cause light to fall off at the bottom if you are tilting it up to the booth, especially in the magnitude of tilt that the Brandt theatres were using.  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 05-12-2004 05:57 PM                     "Good point though I have yet to run across a movie house ( or screeening room) that could afford that kind of tweakeage who wouldn't opt for a turret in the first place." Matt, These people could have had a turret installed but didn't want to go that route... which is way more than twice the cost of the Schneider PC corrector. I will mention that until Schneider corrects its deficiencies with that particuluar PC corrector I will definately steer clear of it. However, at this years Showorst I was able to show its designer and Dwight some of its problems. Its actually made here, not in Germany...so perhaps that spells some of the reasons that its not as good. Mark  |  IP: Logged

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