Film-Tech Forum ARCHIVE: Keystoning

This is topic Keystoning in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by David Stambaugh (Member # 1102) on 01-16-2002, 10:47 AM:

Suppose a large chain is building a new theatre from the ground up. This means they should know well in advance precisely what all the characteristics are for each auditorium: throw distances, the angle from the projector to the center of the screen (assume stadium seating), screen dimensions, etc. I'm assuming that modern optics can be computer-designed based on all this data, so the optics can theoretically then be optimized for each screen. True? If this is the case, then why would there still be some keystoning? Is it not possible to "design out" all the negatives and come up with optics that are near-perfect? Is there a cost-issue, where maybe engineering the optics to 90% perfection costs much less than striving for 100%? Or are they in fact just using standard "parts bin" optics that are not customized at all?

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- dave
Crab juice, or Mountain Dew?

Posted by John Pytlak (Member # 331) on 01-16-2002, 11:04 AM:

Although custom optics can be designed to correct for theatre design flaws that cause keystoning, "off-the-shelf" optics of common design are much less expensive.

At last week's ITEA Seminar Series in Los Angeles, Dwight Lindsay of Schneider Optics announced a new version (3.0) of their very useful free "Theatre Design Pro" software, that helps in choosing lenses and designing auditoriums:
http://www.schneideroptics.com/software/theatre_design_pro/

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John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Cell: 716-781-4036 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion

Posted by Pat Moore (Member # 380) on 01-16-2002, 11:26 AM:

David; this is as complicated a topic as the "focus" thread elsewhere in the forums.

Part of the charm of theatres is that each one is different -- in screen size, viewing and projection angels, throw and so on. To custom design and make optical systems for a particular cinema design is incredibly complicated as well as expensive. Even Multiplex operators that build "identical" auditoriums within cinemas find: 1) they are seldom really identical, and 2) the design needs to change anyway, whether the original design is used once or a few times.

Don't leave with the idea that today's projection optics are assembled from glass pieces out of a bunch of parts bins. These are very complicated optical designs, some of which take months and years to optimize and produce. Both ISCO and Schneider make products for a lot of industry segments, including cameras, medical imaging and other high technology lens requirements -- cinema projection lenses are among the most complicated designs they make.

What we can do is design cinemas within bounds that use incredibly good optics like the Isco Ultra Star Plus and Schneider within their design and performance limits. We can accommodate a certain amount of downangle projection now and minimize to tolerable levels, if not eliminate, keystoning effects now. Stadium seating designs have brought a whole new bunch of challenges to film and image projection.

Pat

Posted by John Pytlak (Member # 331) on 01-16-2002, 11:41 AM:

To add to Pat Moore's posting, IscoOptic ALSO has free lens calculation software available on-line:
http://www.iscooptic.de/english/downloads/indexdown.htm

Isco (and other)lens adapters also allow some "perspective correction" for correcting moderate keystone distortion:
http://www.iscooptic.de/english/products/adaptered.htm http://www.iscooptic.de/english/products/adaptergold.htm

Posted by John Walsh (Member # 168) on 01-16-2002, 12:31 PM:

JP is definitely right; custom optics would be prohibitedly (is that a real word?!) expensive. A well designed auditorium should have practically no keystoning.

People have tilted the screen itself to help reduce keystoning. Both ISCO and Schneider make a device that holds the lens on an angle to reduce keystoning (but it causes other problems.) Cheap lenses used in short throw theaters show a noticable pincushion; Again the ISCO and Schneider lenses do a better job at keeping focus in extreme situations.

The architect must work with the space he's given. If the space is, say, on funky angles because of the other buildings around it, the architect must be creative in squeezing in the required number of auditoriums to satify the business plan. This might mean "less-than-acceptable" scree/projector angles. (The bank may not lend you the money to build if there's not enough auditoriums; a single or too few might not generate enough money. )

Posted by Adam Martin (Member # 641) on 01-16-2002, 03:48 PM:

prohibitively.

It always seems like the building is designed from the lobby outward and the projection equipment is crammed into the leftover places that it shouldn't be.

Example: two auditoriums of the same size back-to-back where the projectors must be placed off-center in the narrow booth so that a human can walk between them, which creates not only keystoning, but reflections off of the port window bounce through the other port onto the other screen.

Posted by Philippe Laude (Member # 79) on 01-17-2002, 01:32 AM:

Pat, I'd like to meet those PROJJECTION ANGELS you're talking about.

Philippe

Posted by John Pytlak (Member # 331) on 01-17-2002, 08:26 AM:

Phillippe said: "Pat, I'd like to meet those PROJJECTION ANGELS you're talking about."

I think they are hanging out with the TRAVEL GHOSTS.

Posted by Steve Guttag (Member # 268) on 01-17-2002, 08:46 AM:

PC-Adapters are great but all of them are for 101.6mm (4") lens mounts so your projector must be able to handle that. Projectors with lens turrets often have the PC adapter's beneifits (if to a lesser degree) built in in the form of adjustable horizontal and rotational stops.

When one uses a PC adatper, they are trading keystone effects for linearity affects (probably the lesser of the two when most people judge a picture).

When on shifts a lens, you have limits on both the entering pupil of the lens and the exiting portion of your lens system (including the PC adapter or turret). This is more of an issue with shorter focal length lenses (as are used in the bulk of "modern" theatres.

That said, Schneider just redesigned their PC adapter so the technology is being supported and is progressing. ISCO uses two eccentric cylinders that can allow about 5mm of shift at any point. Schneider has a slider for the lens tube so one can slide the lens about +/- 5mm (and maybe more). After that, with the Schneider adapter one can rotate the whole adapter to place the lens in the desired location. If one is merely trying to compensate for vertical keystone, then lock the adapter down with the slider in the vertical orientation and adjust until you picture is centered.

For those that have not used PC adapters, you haven't lived until you try to use one with a Scope lens! It WILL try your patience.

Steve

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"Old projectionists never die, they just changeover!"

Posted by Pat Moore (Member # 380) on 01-17-2002, 12:02 PM:

Oops.

Angles, angels and ghosts, OH MY!

What can I say? I've seen a lot more ghosts than angels, that's for sure.

Posted by Philippe Laude (Member # 79) on 01-18-2002, 03:03 PM:

Steve, you're right when you say:

I use an Isco PC adapter with an Isco Blue star anamorphic on a Bauer B11. The major problem is that almost each time I place the lens with its bracket (the B11 hasn't got a turret)it rotates somehow, although I handle it very, very carefully, so the image is a litlle crooked on the screen and I have to correct it.
I'm not sur I could express it clearly, my english being too poor.

Philippe

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Posted by Steve Guttag (Member # 268) on 01-18-2002, 06:11 PM:

Your English is just fine...

How is the lens rotating? Is the ISCO adapter slipping? Or is your indexing ring slipping? I have a modification for the PC-Cine adapter that will stop it from slipping (and not render it useless) but it does require disassembly and some milling. ISCO could have gotten the lock mechanism a bit better on it. Why they haven't corrected the locking design is a mystery to me except they are bound to be low production as compared to other items.

When we design a multi-format booth (beyond the Flat/Scope lenses) then we use PC-Cine adapters quite a bit so all formats are centered on the screen. A recent installation had 11 lenses (5 on one projector and 6 on the other) and PC-Cine adapters are must to get all of those to center up. (the formats supported are 1.37, 1.66, 1.85, 2.39-Scope, Silent and 16mm on one projector).

Steve

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"Old projectionists never die, they just changeover!"

Posted by Carl King (Member # 398) on 01-20-2002, 12:10 PM:

Here's a question for you, John Pytlac. Will keystoning be a problem with digital cinema? I assume so because you will still have odd projection angles.

Carl king

Posted by David Stambaugh (Member # 1102) on 01-20-2002, 12:35 PM:

Keystoning will still be an issue. Some LCD projectors claim to have built-in keystone-compensation. The one below claims "2D Digital Keystone Correction":
http://www.dukane.com/AudioVisual/products/9115.htm

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- dave
Crab juice, or Mountain Dew?

Posted by John Walsh (Member # 168) on 01-20-2002, 12:48 PM:

While the "physics" of keystoning won't change (with digital projection) the projector itself can be simply mounted outside, in the auditorium, on the wall where ever it provides the best picture. It may need to be "blimped" so people don't hear fan noise, but that's far easier than a whole projection booth.

Posted by David Stambaugh (Member # 1102) on 01-20-2002, 12:58 PM:

Doesn't it seem likely though that theatre design won't change much when digital is rolled out widely? The projector is still relatively large and requires lots of cooling for the lamphouse. And won't there be new rack-mounted computer equipment involved? Some kind of server and fault-tolerant RAID hard-drive system for each screen for storing the massive amounts of data. Seems like the booth will need a LAN of some kind too. Would seem impractical to move the projector to a location different from all the other associated equipment (sound rack, automation, etc).

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- dave
Crab juice, or Mountain Dew?

Posted by John Walsh (Member # 168) on 01-20-2002, 06:50 PM:

Actually, I thought the opposite.. the projector can be mounted on the wall, and the video wire for it run to a closet on the main floor where the equipment rack would be. No need to have a projection room at all. So the overall height of the building is lower, you don't have a projection room to heat or cool (although, some places I've worked didn't provide A/C or heat anyway!) A big savings.

Posted by John Pytlak (Member # 331) on 01-21-2002, 07:07 AM:

Today's digital cinema projector designs still use a modified conventional lamphouse, and a large xenon lamp that needs venting. Not likely to be mounted "on the wall". Ultimately, it is likely that the server would be in its' own conditioned room, powered by a UPS, and networked to each projector:
http://www.kodak.com/US/en/motion/digital/system.shtml

Digital cinema projectors are likely to incorporate electronic keystone correction that pre-distorts the geometry of the image to correct moderate keystone.