Film-Tech Forum ARCHIVE: porthole size recommendation

This is topic porthole size recommendation in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

To visit this topic, use this URL:
https://ft-forum.com/ft/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=16;t=001239

Posted by Scott Norwood (Member # 30) on 10-08-2012, 08:01 PM:

Subject says it, pretty much. What size portholes are recommended for D-cinema projectors and how far back should the front of the lens sit from the porthole?

Specifically, is there any reason why a standard 12"x12" porthole would not be sufficient for an auditorium with a "normal" throw ratio (say, 2.5:1 or longer), a non-excessive projection angle, and in an installation without an anamorphic lens?

The only reason why I even bring this up is that most D-cinema projection lenses have significantly larger front elements than film projection lenses. Does that necessitate larger portholes, all other things being equal?

Posted by Manny Knowles (Member # 1171) on 10-08-2012, 11:40 PM:

One thing to watch for: If the porthole size is underestimated, the installer will end up having to place the projector so close to the glass that it will be difficult or impossible to (a) remove the lens; or (b) open the porthole for cleaning.

That is to say, difficult or impossible without moving the projector out of the way.

I've had cause to remove my lens at least twice. And I happen to prefer clean port glass. So those are two very good reasons to have some clearance between the lens and the glass.

Of course, setting the projector back from the glass means -- a bigger glass -- more money. Which means... small, dirty window eventually..?

Posted by Brad Miller (Member # 2) on 10-09-2012, 12:52 AM:

Remember also when trying to make critical adjustments to the projector such as convergence, Scheimfleug, integrator rod, etc...you will WISH you had larger port windows!

Posted by Edward Havens (Member # 4715) on 10-09-2012, 02:26 AM:

Each of our IMAX digital portholes are IIRC about 26in square, while the remaining digital theatre portholes are about 24in square, not counting the secondary opening porthole to monitor the room from the booth.

Posted by Carsten Kurz (Member # 5396) on 10-09-2012, 04:55 AM:

Depends on your specific setup. A standard installation may get away with even smaller portholes. But if you CAN increase the size before installing the machine, it is certainly a good idea. If you want to be prepared for all sorts of 3D options - like Sony 3D or RealD XLS - you should definitly have a large porthole.

As others said, the minimum requirement is that you get an easy access to the porthole for cleaning. And that means, easy access to screws, and space for swinging out the glas. 12*12 is already something. The standard 'old-style' portholes in germany are 5*7 - even that would work with a standard 3D D-Cinema installation, e.g. using shutter glases. But it would be painful for the initial lens adjustment.

- Carsten