Film-Tech Forum ARCHIVE: Projection port distance from lens.

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Posted by Adam Fraser (Member # 1074) on 03-11-2015, 10:46 AM:

We are working with an architect to add another screen to our theatre.

We have a chance to re-use our old booth but there will be a 10 foot distance between the projector and a window in the back of the auditorium. The image will be about 2.2'x5.5' when it gets to the port in the rear of the auditorium.

Any problems we need to be aware of when placing the port window so far from the lens?

Who would you recommend to have make a 3'x7' port window with the proper glass. I know the window will be very expensive but should be much cheaper than making a complete cantilevered projection booth for a 80 seat auditorium.

Posted by Mitchell Dvoskin (Member # 751) on 03-11-2015, 12:10 PM:

There are certainly better qualified people on this forum than I to answer this, but from my experience the 2 issues you will face is loss of light and the image being reflected back off the glass into the lens (which creates strange visible artifacts).

Posted by Monte L Fullmer (Member # 2797) on 03-11-2015, 12:20 PM:

Would it be feasible to shoot through an open port if a "baffle tunnel" of sorts were created between machine and window?

Posted by Frank Cox (Member # 6258) on 03-11-2015, 02:03 PM:

What about something along the line of a big furnace duct, open on the auditorium end and with the port glass placed on the projector's end. If you insulate it, no more sound should escape into the auditorium than usual.

Posted by Dave Macaulay (Member # 813) on 03-11-2015, 03:55 PM:

The light loss is the same whether it's a 4" square or 4'. Object or dirt on the glass look a lot worse on screen with the glass farther away though!
You should use coated optical glass to reduce reflection, and that gets expensive for a 4' piece.
It's better to have the glass tilted back so reflections hit the floor and don't bounce back to the screen: particularly with 3D adapters you will get weird ghost images if uncoated port glass is square to the projector. The reflection off the lens front element is magnified by the convex surface and that glass is coated - I have seen a very very dim ghost from a cross pattern off the lens with uncoated window glass parallel to the lens but it would not be an issue in a movie.
I would recommend using proper coated optical glass with a tilt.

Posted by Rick Raskin (Member # 1561) on 03-11-2015, 04:47 PM:

I like Frank's idea. Simply frame in a light tunnel from the old booth wall to the theater. Place the port glass at the booth end and angle it down as Dave suggests.

Posted by Adam Fraser (Member # 1074) on 03-11-2015, 05:27 PM:

Thank you for the replies, and will run all of this by my architect. The only problem I am seeing with a light tunnel is aesthetics as it would go over our lobby and block the view of a 12' wide x 18' tall stone fireplace.

Posted by Carsten Kurz (Member # 5396) on 03-11-2015, 06:53 PM:

3'x7' is a heck of a port glass. But it is certainly doable. Roughly 2sqm of Amiran. It is available in the necessary format and strength, since it is used for architectucal applications as well. Depending on your supplier, it could cost you 1500-2000US$ excl. mounting/framing.

There are different varieties of Amiran - make sure you get the right one - 'optiwhite'. 'Amiran' only depicts the antireflective coating.

- Carsten

Posted by Steve Kraus (Member # 476) on 03-11-2015, 09:38 PM:

Wait...are you saying the projector beam is going to be going across your open lobby and then through the giant port at the rear of the new theatre? What would keep extraneous lobby light from the theatre or am I not understanding the plan? If so this seems like a lot of trouble to keep the projector in the existing booth and if you really don't want to build a new booth for it then, as despised as it might be, maybe this is a call for a boothless setup.

Posted by Adam Fraser (Member # 1074) on 03-11-2015, 09:59 PM:

The projector has projected above an open lobby for 74 years with no extraneous light problems. What we are looking to do is twin our auditorium horizontally, while building a new booth for the front auditorium.

We will now be building a rear wall to the new auditorium, and the lobby will be more of a common area than it was before.

Here is a picture of the lobby in question, the portholes are on the right. The wall you see on the left will be removed and built to the roof, which would require a port window for the image to pass through.

Posted by Carsten Kurz (Member # 5396) on 03-17-2015, 05:49 AM:

Wow. Hard to imagine you're not getting issues with ambient light spill through a 2qm projection window. Yes, a 'light corridor' across that part will look awkward. Really no chance to get the projector over to the left area/closer to the screen?

- Carsten

Posted by Rick Raskin (Member # 1561) on 03-17-2015, 04:25 PM:

How about if you periscope the image up to the ceiling and then into a light tunnel? I've seen it done in reverse where the booth was on the floor above the theater. I've also seen it done where the ports were too low and patrons in the balcony would have interfered.

Posted by Sam D. Chavez (Member # 1841) on 03-17-2015, 07:40 PM:

There's no good argument for any periscope. The resulting picture is always unacceptable.

Posted by Randy Stankey (Member # 64) on 03-18-2015, 12:39 AM:

Okay... Does a guest would walk along this "hallway" until he gets to the stone fireplace then hang a left to get to his seat?

The projector, in the booth, above the buffalo head shoots across the hallway and over to the screen, which is on the left?

So, then, the picture shoots out of the port window, over the gap and would go through another piece of glass?

Yeah, it can be done. I've seen stranger things.
But I'd have to say that I'm not real hip on the picture shooting through two pieces of glass, much less one that will be hard to reach when it gets dirty.

Use optical glass and angle it and, from what I can tell, it will work, though.

Posted by Mitchell Dvoskin (Member # 751) on 03-18-2015, 09:12 AM:

If you are going to spend the money to do construction, do it right. As impressive as that stone chimney is rising 2 stories up to the ceiling, ultimately your patrons are there to see the movie, not the lobby.

That said, don't build the new wall up to the existing ceiling. Build it to the level just below the existing projection port holes and build a new ceiling from that wall to the port hole wall, leaving the space above it open so you can project the picture over it. I've seen this done, it works fine. I have even seen this done so that from the auditorium, the area above the lobby is made to look like a faux balcony.

Posted by Louis Bornwasser (Member # 3063) on 03-18-2015, 05:53 PM:

<Large port is OK if kept clear. Glass is expensive if good. Some optical glass is not made over a certain dimension. If extraneous light is a problem, a partial light tunnel near or on both sides of the large optical glass will stop it.

I would be tempted to name the auditoriums: Moose and Squirrel.

Posted by Adam Fraser (Member # 1074) on 03-18-2015, 07:37 PM:

Nice idea Louis on the auditorium names!

Thanks for the suggestions again everyone, and will let you know how whatever we decide to do turns out.

Posted by Mike Babb (Member # 1379) on 03-18-2015, 07:51 PM:

Louis, maybe they could go with Roadrunner and Coyote?