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Author Topic: Use of x2 Optic Mirrors to adjust projection output centre Adrian Hauser Film Handler Posts: 44 From: Sydney Registered: Mar 2008 posted 04-15-2008 07:08 PM                        Hi, Im wondering if anyone has successfully experimented with the use of optical quality mirrors to 'centre' the light path of a projector that for whatever reason cannot be placed in line with the centre of the projection screen. Due to reasons have having a 2k Barco in our projection room the 35mm projector has been given the off centre position (RHS threading). This is a bit of a problem in scope mode as we can not get an even focus across the screen. Can I successfully use 2 optical mirrors to rectify this? What are the obvious drawbacks? Adrian  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 04-15-2008 07:31 PM                     Generally, you need to watch yourself with mirrors...We have used them in some venues to move the image to a more desirable position (normally up or down) but at a theatre I used to work, the Janus in Washington, DC, they used mirrors to laterally shift the image. At the Janus it worked okay. I couldn't tell you want the resolution was...the lenses were Kollmorgan but the screens were rather tiny (I mean less than 10-ft tall type of tiny) so on a sizable screen, the flaws in the mirrors would have shown themselves. Have you considered lens shifting? I do many multi-projector installations and I always lens shift to minimize/eliminate keystone. Schneider and ISCO both make lens shifters...aka PC-Cine adpaters (PC = Perspective Change). One thing to also check...make sure your lens is square to the gate...this is especially true with turrets that open/close via hinge...I've seen many become sloppy and allow the lens to skew and cause an uneven focus. Steve  |  IP: Logged Richard Fowler Film God Posts: 2392 From: Ft. Lauderdale, FL, USA Registered: Jun 2001 posted 04-15-2008 07:31 PM                     It can be done if you have the room to create a sideways periscope with two optical grade front surface mirrors of glass or optical grade optical mylar film....The drawbacks, possible image vibration, average 3 - 7% light loss per surface, alinement and maintinance issues. If each machine is not being used at the same time, have you considered a rolling dolly set-up so each unit can be put into optical center when required. I was doing set ups like this 35 years ago and more recent incarnations of this system can be had from Sam at B.A.C.P. Regarding lens shift....I think he is the one with a V-12 projector; if he had a 4 inch lens barrel, a P.C. correcter would be a good option. There are eccentric lens sleeves available to use a 62.5mm barrel lens but they do not offer much offset.  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 04-15-2008 08:00 PM                     quote: Steve Guttag At the Janus it worked okay. I couldn't tell you want the resolution was...the lenses were Kollmorgan but the screens were rather tiny (I mean less than 10-ft tall type of tiny) so on a sizable screen, the flaws in the mirrors would have shown themselves. Steve, There was a write up aboput the Janus Theaters in an old Theater Catalog I think back in the late 60's. They had to move the image laterally because of some verticle support posts in the auditoriums that I belive were on center of where the projection port should have been. An interesting little place. Mark  |  IP: Logged Martin McCaffery Film God Posts: 2481 From: Montgomery, AL Registered: Jun 99 posted 04-15-2008 08:14 PM                        Yes, spent many hours in the Janus in HS and College watching movies in what I think was a converted garage. Pillar right in front of the booth, about midway in the auditorium. Was DC particularly mirror and periscope happy or was this common elsewhere? I remember the Cerebus 3 had an underneath periscope system. You couldn't look at the screen from the booth, you had to look down at the mirror to focus. The Town had a periscope because the booth was in the third(?) balcony and the projector couldn't clear the angle. I seem to remember a few more mirror setups, but not where they were.  |  IP: Logged Adrian Hauser Film Handler Posts: 44 From: Sydney Registered: Mar 2008 posted 04-15-2008 08:45 PM                        Here are the dimensions of our small screening / grading room. Im yet to check the position of the lens although it is not a hinged mechanism or a turret for that matter. One lens at a time on this baby. Screen width = 16ft Distance to screen = 29 ft distance from screen left to projector lens = 6 ft We ideallly need to shift/periscope the output of the projector right by about 2ft yes? Lens = ISCO anamorphic ultrastar f80mm GOLD. NB: I have adjusted the lamp focus/position housing to get a very even 16FL across the screen and used the document in the downloads section to achieve optimum focus with the lens. Will the focus and position of the lamp effect Depth of Field on the projection screen? Thanks Adrian Hauser  |  IP: Logged Randy Stankey Film God Posts: 6539 From: Erie, Pennsylvania Registered: Jun 99 posted 04-15-2008 08:58 PM                     Doesn't the Barco have built-in lens shift and keyston correction? Our Christie does. (Even though we don't have to use it.) It seems to me that it would be more logical to put the film projector at the center and move the Barco slightly off center. 1) You wouldn't have to move it as far off center because it's smaller. (Unless you're talking about a large projector with a xenon lamphouse.) 2) You can correct any distortion in the image with above mentioned keystone correction. Or, are you not using the film projector as often as the video?  |  IP: Logged Adrian Hauser Film Handler Posts: 44 From: Sydney Registered: Mar 2008 posted 04-15-2008 09:10 PM                        Yes the Barco does have built in Keystone correction, (optical and digital I think ) but seeing as we are in such a critical environment whilst grading I refuse to use the digital correction at all as im sure it degrades the image quality however minimal that may be. We grade with the Barco 80 percent of the time and screen mute check prints the other 10 %. With the room dimentions I posted above do you think I should be seeing a focus problem. Adrian.  |  IP: Logged Bill Enos Film God Posts: 2081 From: Richmond, Virginia, USA Registered: Apr 2000 posted 04-15-2008 09:48 PM                     Our Barco DP100 is 10 feet off center and is not going to get the prime spot until digital is our main image source. It shoots down at 14 degrees with a 110 ft throw. It does not have enough keystone correction to compensate, it could be lens shifted until the pic touches the left mask but it would look pretty stupid shifted that far off center. I guess Barco never anticipated anything but the perfect installation.  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 04-16-2008 06:21 AM                     D-Cinema projectors built to DCI specs have no "electronic " keystone correction nor is it allowed. All you can do is shift the lens and the BARCO and all other D-Cinema projectors are capable of that to at least some degree... also depends on how many pixels in size the image is vs. how far you can move the lens and so on. Mark  |  IP: Logged Adrian Hauser Film Handler Posts: 44 From: Sydney Registered: Mar 2008 posted 04-16-2008 06:38 AM                        Im calculating the Film Projector is about 6 degrees off axis to the perpendicular centre of the screen. With the aforementioned dimensions of the room and lens do you think I should be having to move or periscope the Film projector to fix my focus issues. The focus issue being that using the RP45 alignment loop I cant get the focus across the entire scree. Ive read that anamorphic lenses have less DOF and im seing what appears to be the focal plane not running diagonally to the screen. Adrian Hauser  |  IP: Logged

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