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This topic comprises 2 pages: 1 2
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Topic: Off-center projection and keystone mitigation
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Dave Macaulay
Film God
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Posts: 2321
From: Toronto, Canada
Registered: Apr 2001
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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.
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Carsten Kurz
Film God
Posts: 4340
From: Cologne, NRW, Germany
Registered: Aug 2009
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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
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