Aaron

About 6 months after a theater opens, I notice a slight shadow on the top of the screen because (I think) the projector has settled. With the weight (the projection head) in the front, it pulls the front down.

And, (as others have noted) changing or adjusting the xenon bulb puts it in a slightly different position, whih can also cause a shadow.

Masking motor limit switches and rigging shift a little, too.

I always left about 1 foot around the masking (depending on the screen size.) Some people would think that's a bit too much, but I never had to go back and refile. Even with that, I was able to keep cropping equal to or less than 2%.
 

That screen post reminded me of the time I came on shift to find the lateral guide screw on the century had worked its way loose and the film was moving horizontally a bit on screen. On the way up the stairs an usher stopped me and said we had a problem with the screen because the picture kept shaking...

I just looked at him in amazement.

This entire topic of the optics is seriously under appreciated and a major flaw in the installation and maintenance in a lot of plexes.

I have been doing some portable setups for animation and film festivals (lots of Spike & Mike) for a number of years, using 6 lamphouses with two projectors. The subtle changes in the optics mentioned above, with the repetition of sometimes once a week, make it apparent that a lot of new equipment installers don't really understand this part very well.

I have seen a lot of new installation alignments that were close, but the benefits gained by centering the reflector on the arc and directly through the center of the aperture, through the center of the lens, and the center of the screen are substantial.

In addition to the obvious benefits (more light at less amps) the depth of field of focus can be very different depending on the care of the installer.

This is a very interesting subject, one in which there is a lot to learn.
 

In other words, your apertures are "probably" fine, although I do agree that over a significant amount of time, the heat from the lamp will affect the aperture to some degree. Try everything else you can before you go cutting.

Could also be the lens holders and adapters as well. Most adapters have some play in them as they fit in the 4-inch holder and just don't quite register in exactly the same place each time. Clampng the lens in the holder a little more or less tightly can make a difference on screen, depending on the projector and the fit of the parts.

I've found it's best, for instance, to give the lens a little "twist" in one direction as you tighten the holder, that usually gets things lined up in the same place. If the initial alignments of the system are setup the same that usually works fairly well.

Don't forget that just the slightest change in alignment at the projector can make a significant change in the image location on screen. Depending on the throw and lens focal length, change in the projection angle of less than one degree can translate into several inches or more of change at the screen.

Other suggestions above are good as well -- masking stops can drift, masking edges sag, etc. I think the more random things are probably projector-based, though, or there are real problems with the masking motors.

I will say this: re-file apertures only after you know the problem lies there.

Be sure to check film ratios and masking stops using RP40 test film before doing any filing. The aperture opening may require a little buffing now and again to get rid of burnt-on film and oil goo, but if the system was installed correctly by a good tech re-filing an aperture should be among the last things you need to do.

Good luck!

I'm not questioning your knowledge, but since you don't have turrets, is it possible that someone has switched plates and/or lenses? I have never seen an aperature plate "shrink"

Rick
 

I'm just trying to think of any possibilities for the guy.
 

Anything new since?
 

Hope it helps ,
John
 

I doubt that "settling" is a common problem, so look first for other causes. First, be sure your aperture plate and lenses are properly seated in the projector, and haven't been changed (e.g., filed, shimmed) without your knowledge. If the image is still too low on the screen, the adjustment will depend upon whether you have a console or pedestal mount for the lamphouse and projector. Most consoles have adjustable leveling "feet" that can be adjusted to raise or lower each corner of the console slightly. On cast iron pedestals, there is usually a large knob that turns an adjusting screw that tilts the lamphouse and projector back and forth to raise and lower the projection angle. Or, like consoles, the pedestal may have been leveled with four screw-adjusted "feet". Sometimes, shims are used under the "feet" -- perhaps they slipped out of position?

As always, use the SMPTE 35-PA (RP40) test film to verify the image size and position on the screen. The black screen masking should define sharp edges corresponding to the format --- ideally a 0.825 inch image width for all of today's 35 mm formats. Scope projectable image height is 0.690 inches. Flat 1.85:1 projectable image height is 0.446 inches. In reality, unless the projector is perfectly orthogonal to the screen, there will be some "keystoning" that may require slight cropping of the image to get a rectangular image on the screen.

A good way to get a "feel" for what changing various things like throw, lens focal length, projection angle, etc. do, is to use the Schneider Theatre Design Pro software, which can be downloaded from their site:
http://www.schneideroptics.com/theatre/desprodn.html

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com
 

I always take the edge of a file and "notch" the top edge of the plates and scratch in the theater number.