Film-Tech Forum ARCHIVE: Off-center projection and keystone mitigation

This is topic Off-center projection and keystone mitigation in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by David E. Nedrow (Member # 4981) on 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.

Posted by Lindsay Morris (Member # 1430) on 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 [Smile]

Just dive into the lens setting menu and away you go.

BUT do remember to SAVE the settings though. [thumbsup]

Lindsay

Posted by Justin Hamaker (Member # 2165) on 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.

Posted by Marco Giustini (Member # 4544) on 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)

Posted by Carsten Kurz (Member # 5396) on 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

Posted by Marco Giustini (Member # 4544) on 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.

Posted by Dave Macaulay (Member # 813) on 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.

Posted by Carsten Kurz (Member # 5396) on 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

Posted by Sascha F. Roll (Member # 9043) on 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.

Posted by Marco Giustini (Member # 4544) on 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?)

Posted by Ken Lackner (Member # 1002) on 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...

Posted by Scott Norwood (Member # 30) on 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.

Posted by Jim Cassedy (Member # 4115) on 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~

Posted by Steve Guttag (Member # 268) on 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.

Posted by Marco Giustini (Member # 4544) on 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! [Smile]

Had I opted for a British word, it would have been 'pissed' [Big Grin]

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.

Posted by Frank Angel (Member # 248) on 09-22-2015, 11:20 AM:

quote: Steve Guttag
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.

Wow, that sure is good to know. I guess we won't be using Christie when we finally go digital. We have the typical (and all too common) single screen problem -- a booth 5 stories up and projectors at a massive downward tilt -- so severe that there are two-by-fours under the ass-end of the pedestals and we had to put those special Simplex brackets on the XLs' takeup magazines so as to tilt the magazine forward enough that it clears the floor and doesn't get crushed into the floor like a bug when you crank the pedestal tilt wheel down. Our screen, when it flys in, is also tilted bottom forward to compensate as much as possible for keystoning. I guess putting a Christie in would be ill-advised, eh?

Posted by Marco Giustini (Member # 4544) on 09-22-2015, 02:48 PM:

Christie 2210/2215/2208 are ok, lamps are parallel to the screen.

Posted by Louis Bornwasser (Member # 3063) on 09-22-2015, 02:58 PM:

Even if you get a "perfect" picture using lens shifters, the reflection off the screen will suffer just like it did in 35mm. Maybe with digital there is an opportunity to reduce or eliminate being "off."

The goal is to get a undistorted picture AND put the reflected light into the center of the best seating area.

This is even more important (required?) when using silver screens or any high gain screen.

Posted by Monte L Fullmer (Member # 2797) on 09-22-2015, 03:57 PM:

Would tipping the top of the screen back a few degrees help any?

This was an old practice for wide palaces with average throw lengths to reduce keystone being the booth was above the balcony.

Posted by Louis Bornwasser (Member # 3063) on 09-23-2015, 05:51 AM:

Yes. These things will never change. The information may be forgotten, however.

Posted by Frank Angel (Member # 248) on 09-23-2015, 07:25 PM:

Yes, Monte, that's the same as what we do, but we tilt the bottom foward; the top of our frame is chained to the fly pipe. Same effect, you are compensating a bit for the keystoning. But of course, with these severe angles, you can't fully correct it, nor would you want to, given you would then be directing much too much of the light up into the ceiling instead of the audience. The only way you can get away with this cheat is if you have a matte, 0-gain screen. I can't imagine what a horror-show you'd create trying to do that with a silver screen or even a mild gain screen.

I am afraid if we want 3D in this house, we are going to have to go with Dolby or it's 2D only.

There is also the Omega color interference system -- same as the Dolby concept, only they use a 5 band color comb filter for each eye instead of the 3 that Dolby uses and that eliminates needing to color correct the two images. BUT, their downfall is that this system is only available for a dual projectors. They've yet to offer a filter wheel; you have to have two projectors or nothing. On the plus side, their glasses are much less expensive than Dolby's. Not much of an economic relief if you initially have to buy a second projector. It would be a great system if they got their act together and offered a filter wheel for a single projector system.

Posted by Stephan Shelley (Member # 8587) on 09-23-2015, 07:51 PM:

There is also the expand system that uses the active glasses. No silver screen needed. Much more light efficient than the Dolby one. Glasses are even more expensive than the Dolby ones. And have batteries too.

Posted by Frank Angel (Member # 248) on 09-23-2015, 08:12 PM:

That ought to make for an intersting budget proposal for our 2500 seat house. [Eek!]

Posted by Stephan Shelley (Member # 8587) on 09-23-2015, 09:17 PM:

Even the Dolby glasses at around $10.00 each is good budget item.

Posted by Steve Guttag (Member # 268) on 09-24-2015, 06:35 AM:

Ah but the Dolby (or XpanD) glasses are a recycle item (so you need to thing of the washing equipment too. That said, if you do the math on their use life cycle, it is a pretty cheap 3D show. However, with a 2500 seat venue, you also need to RECOVER those glasses (more so with XpanD) or running 3D gets VERY expensive.

Note too, with the 6P laser projectors, you have your option of single projector (Barco) or dual projector (Christie)...then the Dolby light efficiency shoots to the top of the pack since no filter wheel is required (and sucking light...filters only take away, never add).

But seriously...3D? How many anticipated shows would you have to justify any system?

Posted by Frank Angel (Member # 248) on 09-24-2015, 12:39 PM:

Not enough to justify even suggesting such an expense. We are still floundering being able to justify installing digital at all. It's just that if I were king and could wave my hand and get anything I wanted, of course I would I'd want a booth to be capable of running most any format. You don't want a booth that can't run everything from 16mm with our trusty Eastman 25B to 70mm to digital. I certainly would not want to have to say, oh we can't run a 3D title if a film professor came to me and wanted to show THE HOUSE OF WAX to his class (or I wanted to have a pizza and movie party and show AVATAR to the staff for one of our private midnight shows as we have every so often). Look, I wanted to swap out our XLs to JJs but couldn't honestly even justify that expense as much as I wanted it so badly. Everyone has to live with the limitations you're dealt.

Posted by Monte L Fullmer (Member # 2797) on 09-24-2015, 02:29 PM:

quote: Steve Guttag
But seriously...3D? How many anticipated shows would you have to justify any system?

(Can we add Barco Escape into this equation?)

Posted by David E. Nedrow (Member # 4981) on 10-02-2015, 07:43 PM:

I think we're now at the point of deciding whether to expand the projection platform slightly or build a projector enclosure in the theatre itself. The latter would allow the projector to be on the centerline horizontally, and closer to the vertical centerline. And maybe less expensive that the platform alternative.