Film-Tech Forum ARCHIVE: Critical distance between projector in change-over booth

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Posted by Marin Zorica (Member # 1716) on 02-22-2010, 06:56 AM:

I have one booth, screen is 8m (26ft) wide for scope, distane is 26m (85ft), and projector are each by side from center 1.9m (6ft), so distance between projector are 3.8m(12ft) total.

I did check on schneider software that distortion is 2%.....what you think on this? It would be okay or....also screen is flat.

Posted by Ian Parfrey (Member # 5122) on 02-22-2010, 12:27 PM:

Marin.

2 % is an abstract value that I find hard to correlate with image quality. How does the picture look with some RP40? If the images on that is ok then all should be fine.

At a throw of 26m, the closer together you can get the projectors the better but then you have practical issues to deal with such as risk of bumping the right hand machine when threading the left and so on. Somewhere there will be a happy medium where distortions are acceptable (if at all) and operator convenience is not compromised.

Posted by Monte L Fullmer (Member # 2797) on 02-22-2010, 12:40 PM:

? have you seen the spread between projectors in Brad's new booth for his screening auditorium ?

... and I bet that's not even a 50ft throw.

Thus, with an 85ft throw, 12 ft isn't too bad - maybe a light case of keystoning.

-Monte

Posted by Hugh McCullough (Member # 1542) on 02-22-2010, 05:04 PM:

In the UK, irrespective of the length of throw, the recommended distance between change over projectors is 4ft 6ins, about 1.4 metres. This is measured from the optical centre of projector 1 to the optical centre of projector 2.

Posted by Marin Zorica (Member # 1716) on 02-22-2010, 05:08 PM:

That's 2.8 meter from both projector, we have here about 1.8 to 2m mostly....well the less the better and less space [Big Grin]

As, screen is smaller and distance is higher, the distortion is less...

Posted by Brian Guckian (Member # 1678) on 02-23-2010, 01:38 PM:

quote: Marin Zorica
I did check on schneider software that distortion is 2%.....what you think on this? It would be okay or....also screen is flat.

Yes, it's OK. The horizontal offset from the screen centreline of each projector appears to be 4.18 degrees approx., calculated thus:

Inverse Tan 1.9 / 26 = 4.18 degrees

The usual rule of thumb is to keep the offset under 5 degrees maximum.

This is only a coarse measure and the distortion equations are far better - 2% is very good (3% is usually the maximum preferred value, and not to exceed 5%).

It also looks as if you could accomodate another machine in a central position, based on your information.

Posted by Jim Cassedy (Member # 4115) on 02-23-2010, 05:04 PM:

The old "Mitchell's Projection Manuals" had a simple formula
for determining recommended spacing between dual projectors
based on the distance to the screen. Unfortunately, I can't
seem to find my manual at the moment so I can't look up the
info for you, but I'm sure someone else on the forum probably
has one of these books and can look it up.

Posted by Brad Miller (Member # 2) on 02-23-2010, 09:14 PM:

Monte, it also required quite a bit of custom modification to the projectors to keep the entire image in focus.

Posted by Scott Norwood (Member # 30) on 02-23-2010, 09:29 PM:

In case anyone cares:

The old SMPTE projection book has a diagram which specifies 72" from lens center to lens center.

Massachusetts code requires 42" clearance on each side and rear of each projector, although it is not clear if the spaces can overlap. (I assume that this is allowable, as I have never seen a booth with 84" of space between the projectors.)

One annoying "feature" of at least two newish (last few years) installations in this area is the placement of a video projector between the 35mm projectors. I can almost understand why this was done, but it rubs me the wrong way to think that film presentation is being compromised for the sake of video.

Posted by Marin Zorica (Member # 1716) on 02-24-2010, 05:02 AM:

quote: Brad Miller
Monte, it also required quite a bit of custom modification to the projectors to keep the entire image in focus.

Can you tell us a bit more detail? You thinking on adjusting lenses a bit, or film path in gates?

Posted by Brad Miller (Member # 2) on 02-24-2010, 11:11 AM:

Lens holder and gate had to be skewed.