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Author Topic: Off-center projection and keystone mitigation David E. Nedrow Master Film Handler Posts: 368 From: Columbus, OH, USA Registered: Oct 2008 posted 09-12-2015 10:54 PM                        A friend is getting ready to switch his final auditorium to digital. The problem is that the existing 35mm projector is 7-8° off the center line of the screen and has a forward tilt of about 20-23°. The throw is about 45′, with a 35′ wide screen with adjustable side masking. I've never watched a film is this auditorium, but this must cause some amount of noticeable keystoning, right? How well can digital projectors deal with this? Is this setup worth changing? He's going to be doing some construction, so he could perhaps make changes to the position of the new projector. Maybe he's a candidate for building an enclosure on the back wall of the auditorium to hold the projector. Opinions please.  |  IP: Logged Lindsay Morris Expert Film Handler Posts: 233 From: Darlington, WA, Australia Registered: Sep 2002 posted 09-13-2015 03:35 AM                     From my limited experience with Digital gear (NEC 1600C & Eiki home type projectors the digital beasts are far superior to the 35mm gear. NO filing of aperture plates required Just dive into the lens setting menu and away you go. BUT do remember to SAVE the settings though. Lindsay  |  IP: Logged Justin Hamaker Film God Posts: 2253 From: Lakeport, CA USA Registered: Jan 2004 posted 09-13-2015 04:01 AM                        NEC projectors do not have any keystone correction. I'm not sure about the other brands. However, the projectors do have horizontal and vertical shifts which can move the image left or right. Plus the screen files have crop settings so you can at least provide a square picture. If it's possible to do so, one solution might be to move the position of the port window. Lower it and move it more toward the center. We had to do this with one of our auditoriums. Because the sound proofing is not as critical with no film chatter, you can do somethings that would not be practical with film. Another possibility might be to create a platform for the projector in the auditorium. Since you don't need to actually touch the projector for basic operations, this might be a viable possibility.  |  IP: Logged Marco Giustini Film God Posts: 2713 From: Reading, UK Registered: Nov 2007 posted 09-13-2015 05:01 AM                     Digital cinema projector do not have keystone adjustment - it's a DCI requirement. You can indeed use lens shift before you rake the machine. Someone (France) don't like that because using the lens shift will end up using the outer edge of the lenses, causing the focus and brightness uniformity to compromise a little - it also depends on the lens being used and the amount of lens shift. Personally I believe the keystone is a far bigger issue than anything else, so I don't see why you shouldn't use it. That said as a rule of thumb if your projector is located ABOVE the top of the screen, you will have to rake the projector and keystone will be noticeable when masking does not apply. If you are going to rake your projector, make sure you end up with a projector which allows you to adjust the focus evenness (Scheimpflug, Boresight)  |  IP: Logged Carsten Kurz Film God Posts: 4340 From: Cologne, NRW, Germany Registered: Aug 2009 posted 09-13-2015 06:21 AM                     Lens shift will also cope with a projector being located above the screen top. Depending on lens and conditions, up to something like 25%-40% screen height. Which is a lot. Some projectors limit the amount of V- and H-Shift already in software so that you can't go beyond certain amounts of shift that would cause image degradation. From my experience, 7-8 degree of center is no issue at all with the lens type needed for this throw. He and you will be amazed how perfect the image will be aligned using lens shift, masking, and such. - Carsten  |  IP: Logged Marco Giustini Film God Posts: 2713 From: Reading, UK Registered: Nov 2007 posted 09-13-2015 07:29 AM                     I doubt you can use so much lens shift without introducing shadows on the picture. That said, it may be better to have shadows than keystone. It's a matter of personal preference.  |  IP: Logged Dave Macaulay Film God Posts: 2321 From: Toronto, Canada Registered: Apr 2001 posted 09-13-2015 10:06 AM                     All digital projectors have a lens shift. Barco projectors set the maximum shift available depending on the lens installed, so they don't allow it to go into lens vignetting. Some Barco lenses allow an amazingly large shift, some are very limited. I haven't seen any serious light or focus dropoff when coming close to the x/y limits. Doing a lens home/return shows you the actual hardware limits of the lens mount, this is usually well beyond what the lens shift adjustment allows for the lens used. For other projectors, I don't know if they have this software restriction. I would set the projector level and square to the screen and try lens shift to see if it can put the image on screen. If it does, check light and focus uniformity. The boresight/ Scheimpflug adjustment should be pretty good as it's set at the factory for square-to-screen. Side to side should be no problem but 20 degrees down is probably too much. Use the minimum projector tilt to get on screen with the lens shifted most of the way down. You will need to adjust boresight/Scheimpflug with a tilted projector. Notes: Don't have the shift at the end of downward travel, stay a tad short. This avoids errors when the unavoidable inaccuracy wants it a tiny bit lower and it hits the limit. Don't save the position after moving the lens away from close to a stop. For better accuracy, projectors want to go to a saved position the "same way" as was saved from: no problem if saved approaching a limit but they go past and come back if saved moving away from a limit... hitting that limit and throwing a "requested lens position not reached" error. Some keystoning has been normal in almost all cinemas. I have seen severe keystoning where credits zoom in from (or past) the screen edge as they crawl up. It's not very noticeable in the actual movie. Set the electronic masking to suit the screen. D-cinema "rules" disallow any scaling, which of course includes keystone correction. Scaling is available in all TI hardware though, you just have to know where it's hidden. Like many other DCI rules, this one gets broken regularly - and nobody from DCI is policing them. The resulting image artifacts are annoying only to the very few patrons who notice them. I do avoid using any scaling, using it only when an owner demands it (and accepts the image degradation). I see systems with it used fairly often though, especially on Christie projectors with manual lens adjustments.  |  IP: Logged Carsten Kurz Film God Posts: 4340 From: Cologne, NRW, Germany Registered: Aug 2009 posted 09-13-2015 10:28 AM                     For 0.98", 1.2" and 1.38", there are so many different lenses, I have never seen a decent table of allowable lens shift ranges. For the more closed S2k series projectors, e.g. NEC has a nice one in this PDF: http://www.nec-pj.com/cat_data/pj/WDPJ-1307-583NN.pdf And you will notice that even these 'tiny' lenses allow a V-Shift of up to 0.5. Yes, that is 50% screen height up or down! At the same time, H-Shift is a mediocre 0.11, and I ran against that limit a few times already. These are actually software imposed limits - during installation I noticed that electromechanically, the lens can travel a lot further, but once you home the lens, the software restricts actual adjustment severely. I actually wish there would be an override mode, because sometimes you just need a tiny bit more, and I hate skewing the projector. - Carsten  |  IP: Logged Sascha F. Roll Expert Film Handler Posts: 140 From: Berlin, Berlin / Germany Registered: Sep 2015 posted 09-13-2015 11:27 AM                     And please notice that besides the poorer Contrast Ratio all DLP 4k machines I know have very limited Lens Shift Capabilities compared to their 2k counterparts. The 4k machine with the widest Lens Shift range is the SRX220/320, followed by SRX510/515.  |  IP: Logged Marco Giustini Film God Posts: 2713 From: Reading, UK Registered: Nov 2007 posted 09-13-2015 12:30 PM                     Keep in mind that Sony machines do allow you to abuse the lens shift till the lens drops on the floor but that does not mean that you won't pay the consequences. Pay particular attention to vignetting and focus issue. I believe Sony allows +/-35% vertical and 7% horizontal (may be mistaken) with those lenses. After all, 35% is just 1.5m on a 10m scope screen (4.1m high). Which means that your projector will have to be 55cm IN from the top of the screen before you have to rake it. (is this calculation correct?)  |  IP: Logged Ken Lackner Phenomenal Film Handler Posts: 1907 From: Atlanta, GA, USA Registered: Sep 2001 posted 09-14-2015 08:42 AM                     I've learned a lot of foreign counterparts for common terms we use by reading the forums here. "Lace" a projector instead of "thread," projection "box" instead of "booth," etc. What does rake mean? Never heard that one before. If it means tilt, why not just say tilt? If it doesn't mean tilt, I haven't a clue...  |  IP: Logged Scott Norwood Film God Posts: 8146 From: Boston, MA. USA (1774.21 miles northeast of Dallas) Registered: Jun 99 posted 09-14-2015 08:50 AM                        One issue not yet mentioned is the maximum tilt angle. At least with Christie (I do not know about other brands), there is a maximum tilt angle (15 degrees, I think), beyond which you will void the warranty. Be sure to check into this before buying/installing the machine. The workaround is a periscope setup, but that can introduce other issues and is best avoided if possible. Other comments about setting up the machine square to the screen, (almost) maxing out the lens shift adjustments, then tilting/shifting the machine and doing the boresight adjustment are dead-on. The projector works like a large-format camera in reverse (with more limited lens movements); these adjustments should come naturally to anyone with experience working with 4x5 or 8x10 cameras.  |  IP: Logged Jim Cassedy Phenomenal Film Handler Posts: 1661 From: San Francisco, CA Registered: Dec 2006 posted 09-14-2015 10:09 AM                     "Rake" is a term used to describe the angle of a slope, or a deviation from either a perpendicular or horizontal plane. That term is quite commonly used by architects, landscapers and engineers here in the USA. I've heard it many times, so I'm not sure it's "British thing'. "England & the United States are two countries separated by a common language" ~George Bernard Shaw~  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 09-14-2015 10:09 AM                     See now, I've heard/used rake and Lace for thread plenty. Never used "box" for the booth in the US though. As for tilt vs rake...why not call it pitch? There are going to be variations on such things from region to region without even leaving a country. With Christie, the tilt of the projector has an effect on the xenon lamp since it is tilting along with it. With NEC and Barco, tilt does not since the lamp remains parallel to the floor. As for toe-in and lens shift...it really is lens dependent. Some of the Fujinon lenses for the Barco "C" series have almost no lateral lens shift capability and you will have a visible keystone in the credits. Remember too, you are putting a rectangle through a round hole. As you shift in one direction, you are necessarily taking away from another so you have to decide what is worse. For 4K, there are now proper 4K lenses out there so one has more shift range than we used to. Note on the construction of those the larger inlet pupil. There is no free in this world...with shift one gets stretch and asymmetric resolution/focus. And yes, you can try to mitigate that to some extent but there is no substitute for being as close to center-line as possible.  |  IP: Logged Marco Giustini Film God Posts: 2713 From: Reading, UK Registered: Nov 2007 posted 09-14-2015 04:08 PM                     Ken, I'm definitely not the right person to debate about English/American language but 'rake' is in fact in the English American dictionary! Had I opted for a British word, it would have been 'pissed' It's a shame to see so many compromises today. Because we can use lens shift, projectors are installed farther and farther away from the centreline. Then you end up using all the available lens shift, causing some vignetting and focus/resolution issues. On top of that, let's put one of the newer 3D devices that claim to be so light efficient (but won't align very well when the projector is raked, I mean tilted - and let's not forget the high gain silver screen for best results.  |  IP: Logged

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