Film-Tech Forum ARCHIVE: Achieving uniform focus

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Posted by Ken Lackner (Member # 1002) on 05-09-2004, 01:02 AM:

If your projector is tilted at an angle towards the screen, is it ever going to be possible to acieve a perfect focus top to bottom? I notice a lot of times I can get the bottom in focus but as I look closer to the top the picture becomes less focused. I assume this is due to the projector being titled. Am I just going to have to accept this as a fact of life?

What about curved screens? I would think that it would be hard to acheive the same focus all the way across. Am I right? (I haven't looked, I'm just asking about that one because I'm curious how it works.)

Posted by Mark Gulbrandsen (Member # 72) on 05-09-2004, 07:53 AM:

Ken,
We need more information...projector type, lenses, screen size, throw, tilt angle, etc., in order to help you.

Mark

Posted by Gordon McLeod (Member # 33) on 05-09-2004, 01:47 PM:

It will usually entailing shimming the lens mount and irising down the lens if it is very steep

Posted by Scott Norwood (Member # 30) on 05-09-2004, 10:02 PM:

The keystone/focus issue can be fixed in a couple of ways. First, the screen frame can be angled to match the projector tilt angle. This results in a properly focused image without keystone distortion when viewed from the booth.

Another option would be something like the Schneider "Cinelux PC Keystone Correction Adapter" which I recently found out about. It's the product that I have been wanting for years. The concept is the same as that behind the use of movements with a view camera to eliminate perspective distortion. The projector would be mounted such that the film runs parallel to the screen (no tilt) and then the lens itself would be shifted to position the image correctly on the screen. I assume that this is useless for extreme tilt angles, but might work nicely when limited correction is required. Hopefully someone who has used these can give more detail on their effectiveness.

Posted by Mark Gulbrandsen (Member # 72) on 05-09-2004, 10:08 PM:

Ken,
Again more information would be helpful as there are adjustments in some projectors that if set wrong could cause your problem. You need to know if its a problem caused by excessive down tilt or by a mis-adjusted projector, or a worn part.....

Scott Norwood said....
"Another option would be something like the Schneider "Cinelux PC Keystone Adapter" which I recently found out about. It's the product that I have been wanting for years."
_________________________________________________________________

Well, its been available for at least 15 years and probably longer. ISCO also makes a PC corrector thats about half the price of the Schneider. I recently installed one of the new black Schneider PC correctors and its a piece of crap. The little adjuster to make the inner barrel shift back and forth is not worth anything and I ended up taking this part out of mine. And when you loosen the clamp screws to allow adjustment the inner barrel then sags down from the weight of the lens making fine adjustment really fustrating. The better of the two is definately the ISCO and oddly enough Schnelder used to make one exactly like it...why they went to the new design I'll never know.

Mark

Posted by Richard Fowler (Member # 893) on 05-09-2004, 10:17 PM:

I used the older Schneider PC correctors in a screening room which had a building support in the middle of the booth wall and the machines where 12 feet apart for a 55 foot throw...no problems. the only problem would be two - three service calls a year because some new operator would shift the projectors since they where "pointing" in the wrong direction [Confused]

Posted by Adam Martin (Member # 641) on 05-09-2004, 10:31 PM:

A misaligned lamphouse can also cause focus problems.

With modern lenses, screen tilt/curve and projection angle should have little effect on focus as long as the lens is properly aligned with the film plane.

Posted by Carl Martin (Member # 1146) on 05-10-2004, 03:24 AM:

quote: Scott Norwood
The projector would be mounted such that the film runs parallel to the screen (no tilt) and then the lens itself would be shifted to position the image correctly on the screen.

could one not achieve the same result by cutting the aperture high in the plate and perhaps adjusting the position of the lamp?

carl

Posted by Michael Schaffer (Member # 1204) on 05-10-2004, 04:45 AM:

quote: Scott Norwood
First, the screen frame can be angled to match the projector tilt angle.

Then you are sitting in an auditorium watching the tilted screen. In my experience, this is far worse than having a little keystoning on the screen since your eye adapts to that while the tilted screen is simply an odd element in a room where you have other visual references.

Posted by Matt Zeiner (Member # 1914) on 05-10-2004, 06:01 PM:

I'm with you, Michael. If there is keystoning, there is keystoning. Thats all. I have fiddled around with a few of those correction gizmos - they kinda work if the keystoning is not too severe, in which case, why bother with it at all. The only time I could see justifying keystone correction is for 16mm blowups. A well-cut aperture will mask minor keystoning. If the keystoning is unacceptable, the building is unacceptable. Perfect is nice but generally unattainable with band-aids.

Posted by Mark Gulbrandsen (Member # 72) on 05-10-2004, 06:39 PM:

The maximum correction with a PC adaptor is about as though you are 7 degrees off dead center. The other good use is for perfectly aligning all the lenses for different formats in a single barrel machine.

Mark

Posted by Matt Zeiner (Member # 1914) on 05-12-2004, 06:14 AM:

quote: Mark Gulbrandsen
The other good use is for perfectly aligning all the lenses for different formats in a single barrel machine.

Good point though I have yet to run across a movie house ( or screeening room) that could afford that kind of tweakeage who wouldn't opt for a turret in the first place.

Matt

Posted by Steve Guttag (Member # 268) on 05-12-2004, 08:39 AM:

Matt,

You don't run across that many screening rooms...obviously. I have literally 100s of PC adapters out there. The ONLY place I've installed with turrets are for main-stream cinema. Most the screening rooms I deal with have 4-5 lenses per machine (1.85, 1.66, 1.37, Scope, Silent and then there may be one or two lenses for 16mm). Turrets don't exactly work there.

We just installed PC adapters in Ambler, PA to deal with some extreme keystone issues as well.

The PC adapter gives the experienced tech a lot of control over the picture placement and appearance. ISCO's will shift as much as 5mm in any direction...Schneider's new one will shift a lot more but it is more pricey. However, when dealing with Silent/super-35 or with long focal length lenses they are quite valuable tools.

However, with that being said...in the typical Flat/Scope theatre, they are rarely necessary.

Steve

Posted by Frank Angel (Member # 248) on 05-12-2004, 08:48 AM:

In the old Brandt theatres on Forty-Douch Street in Manhattan, most of them, like the Victory, the Amsterdam, etc. had booths sitting up 5 and 6 stories above the stage and with very short throws. The angle was insane. All these theatres had fly-hung screen frames on the first or second pipe of the stage (they originally all did vaudeville stage shows along with films, and hence they were designed very shallow so actors' voices would carry to the back of the theatre). The solution all those theatres used to deal with severe keystoning was to simply grab the bottom of the screen and pull the sucker downstage as much as needed to get it fairly close to being parallel to the projector lens. The keystoning would have been terrible without doing that. It didn't correct keystone distortion completely (the damn screen would have had to be laying practically flat on the stage for that), but it was reduced enough to become livable. And that tilt in the screen, from a young audience member like me, looked kinda cool. Sort of reminiscent of the CinemaScope curve, which it turns out, some of those old houses still maintained WITH the tilt.

In my theatre, the booth is 5 stories up, but the throw is much longer (I think about 120ft) so that minimizes the keystoning error, but we still needed to get a hold of special brackets for the take-up magazines which thrust the magazine forward so it does not hit the pedestal. Without it, you can't get the angle you need without crushing the magazine against the pedestal base as you crank the tilt adjustment wheel. I also had to get a roller from Kelmar to push the take-up belt so it didn't rub against the Simplex soundhead casing.

Interstingly, we tilt the screen DOWN toward the middle of the orchestra, not up toward the booth (which, in fact, INCREASES the keystone error). We do this simply because we need as much light as possible to be reflected back to the center of the orchestra, NOT back up to the top of the balcony. We can live with the keystoning which is properly masked away by the aperture plate and the masking, but we can't loose all that light to the heavens. Living with some keystone distortion in the image geometry (but still maintaining a perfectly rectangular looking screen) is the much lesser of two evils.

Of course none of this screen tilting chicanery will work on anything but matte screens surfaces. Any gain screen will cause light to fall off at the bottom if you are tilting it up to the booth, especially in the magnitude of tilt that the Brandt theatres were using.

Posted by Mark Gulbrandsen (Member # 72) on 05-12-2004, 05:57 PM:

"Good point though I have yet to run across a movie house ( or screeening room) that could afford that kind of tweakeage who wouldn't opt for a turret in the first place."

Matt,
These people could have had a turret installed but didn't want to go that route... which is way more than twice the cost of the Schneider PC corrector. I will mention that until Schneider corrects its deficiencies with that particuluar PC corrector I will definately steer clear of it. However, at this years Showorst I was able to show its designer and Dwight some of its problems. Its actually made here, not in Germany...so perhaps that spells some of the reasons that its not as good.

Mark

Posted by Steve Guttag (Member # 268) on 05-12-2004, 06:24 PM:

My biggest beef with the Schneider PC adapter, other than the cost is the poor method of clamping the lens. I have had scope lenses drift ever so slowly out if they are stored on end.

Steve

Posted by Mark Gulbrandsen (Member # 72) on 05-12-2004, 09:41 PM:

Steve,
I assume you mean the black PC corrector......

My beef is that when the clamping screws are loosened a scope lens, ok any lens, will sag downward making alignment a very trying thing to achieve. The little adjustment eccentric is also a waste of time and money as any very fine setting with it is impossible to acheive.... one touch and the pictures in an entirely different placec!!

Mark

Posted by Ken Lackner (Member # 1002) on 05-12-2004, 11:46 PM:

I wasn't really looking for specific answers, it was just a general question. We have Simplex 1060 projectors in some houses, and Century SA's in others. The lenses are Schneider. The screen size, throw, and angle will be different for each house, and I don't know that information.

[ 05-14-2004, 01:16 AM: Message edited by: Ken Lackner ]

Posted by Michael Schaffer (Member # 1204) on 05-13-2004, 12:27 AM:

quote: Ken Lackner
We have Sinplex 1060 projectors

That sounds like equipment for a multi-screen porn house!

The theatres Frank describes sound like they probably had very steep stadium seating. If you are sitting above the screen looking down, that is OK I think, but not if you are sitting lower than the screen looking up.

Posted by William Hooper (Member # 71) on 05-13-2004, 03:58 AM:

quote: Michael Schaffer
The theatres Frank describes sound like they probably had very steep stadium seating. If you are sitting above the screen looking down, that is OK I think, but not if you are sitting lower than the screen looking up.

They're old legit theaters ca. turn of the 20th century that had projection equipment added. Most vaudeville houses (for example, Lansburgh's Orpheums) pretty much were of similar plan.

I can't find a decent section of one online, & seating charts are usually distorted in scale. There's a sliver of the New Amsterdam visible in a picture at its site here .

Typically they're spoken of as following "European opera house" design, but their design evolution in the US was pretty specific. The best description I heard was from Rene Brunet who called the type "a vertical theatre" or "vertical hall" (now I hear & see it in use in a lot of places.

Originally built for stage productions, the seating on the floor was not as deep (still spacious, though), & they had 2 or more balconies (loges, mezzanines, people will call them what they want even though loges & mezzanines have other, specific meanings). The balconies were fairly steep, but it worked out as very good for proximity to the stage & sightlines, resulting in very good, intimate seating even as you got up to the top balcony.

Overall they were more square or undersquare than what the theater & movie palace plans evolved to. By the end of the silent era, big movie theaters with balconies tended to be longer with a sea of seats on the floor in front of the balcony, & with a less steep balcony. Smaller theaters were appreciably longer than wide, too. The reason can be guessed from previous discussion here: for film, less keystone & more straight-on view of the screen in the balcony.

(European design, which was actually based on English & European design & was what US theaters followed in the 18th & 19th centuries, was not as vertical or intimate. The construction was a buncha seats on the floor, that whole area of floor seating called the pit, then a row of boxes running around the back & side walls, & then usually galleries above the boxes. The galleries & boxes at the back did not come so close & were not as intimate with the stage, as the large 'pit' a.k.a. floor seating was mostly in between. In the US, the 'vertical' design evolved as pulling balconies more & more over the 'pit' or floor seats, cranking the balcony angle, stacking 'em up.)

Going from something like the New Amsterdam or aone of Lansburgh's Orpheums (pre-film vertical theaters built for live productions, legit or vaudeville), to the Chicago (1921, built for film, not quite as vertical, but still wider than long) to houses at the end of the silent/movie palace era (spectacular things like the Los Angeles, UA, or spectacular late-20s Loew's & Paramount theaters), they stretched out & squashed flatter. The longer they get, though, the less enjoyable they are for live shows, until you get to some which have the show a block away from the back rows.

There's an excellent middle period particularly of Thomas Lamb's ca. 1926 theaters that are smack in the middle & to drool over for live or film use. These were things like those of Thomas Lamb's with the later Adamesque interiors (all very handsome & formal) before his sudden design freakout of spectacular things like the Loew's Jersey, 72nd Street, Louisville, etc. which by then were deeper & shorter.

So the older, wider-than-long theaters did resemble stadim theaters in plan as you noted, but not in section. The projection booth was put wherever, usually winding up in the top balcony.

Some built-before-movies theaters had projection booths which moved several times from the 1st balcony, to second, to top during their film days).

Hey Frank, if they haven't completely sealed the Liberty for renovation yet, see if you can absentmindely wander in like you're lost & get a picture of that lovely projection booth it had last installed. Asbestos panels, hung from the ceiling.

Posted by Ian Bailey (Member # 1752) on 05-13-2004, 08:25 PM:

Adam
I just can't understand how a mis-aligned lamphouse can affect focus.I would assume it would effect the amount of light on the screen but can it cause a focus problem like I have now in my small auditorium where the picture is out of focus in the right-hand bottom corner compared to the rest of the screen?

Posted by Ken Lackner (Member # 1002) on 05-14-2004, 01:15 AM:

quote:
That sounds like equipment for a multi-screen porn house!

My bad. Post edited to correct typo.

Posted by Gordon McLeod (Member # 33) on 05-14-2004, 09:37 AM:

Ian said "Adam
I just can't understand how a mis-aligned lamphouse can affect focus.I would assume it would effect the amount of light on the screen but can it cause a focus problem like I have now in my small auditorium where the picture is out of focus in the right-hand bottom corner compared to the rest of the screen"

The optical train of the projector is two have much like the way the sound system has an A and a B chain

The lightsource and its optics project a convergin beam of light at the apperture that converges at a distance in front of the apperture and i a pin hole was placed in the lensmount you could focus a image at a distance dependent on the reflector alone

The projector lens also has a point of convergance depending on its focal length and the axis of these two systems must be in line for either to work properly. Certain lens / reflector combinations are more sensitive than others

Posted by Ian Bailey (Member # 1752) on 05-14-2004, 07:39 PM:

Thanks for the information Gordon. [thumbsup]