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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.

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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.

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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"

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