Film-Tech Forum ARCHIVE: pre-focused film gate

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Posted by Thomas Jonsson (Member # 1902) on 08-06-2004, 05:00 PM:

A couple of years ago I read an article by
mr Glen Bergren,who suggested pre-focused
film gates, to compensate for the films
slight turning to the left (or right, I
don´t remember). This is caused by the films
buckling from the heat, and the fact that
the picture is not in the centre of the film,
as the right part is occupied by the analog
soundtrack. According to mr Bergren, a pre-
focused gate should quite improve screen focus.

Has any pre-focused film gates ever been
produced for any projector?

Thomas

Posted by Mark Gulbrandsen (Member # 72) on 08-07-2004, 09:23 AM:

Thomas,
I think what you mean is a shimmed film gate for best flat focus. Glenn Bergren was bestowing this benefit a number of years back. It is true and it works. When I built the VistaVision projector I aligned the film trap very carefully and put small adjustment screws in the foot of the trap to be able to tilt or just slightly angle the trap. I did this alignment with a .0001 dial indictor with the main casting and indicator resting on a 24X36 granite tool plate. The trap was dialed in for absolutely no error at the four corners of the solid runner. This tedious procedure paid off later on when an ILM camera man told me that this VV projector had the best overall focus of any that he'd used, this was to me definate proof that shimming the trap does work! Shimming can also be done to compensate for down tilt and other oddities such as poor machining of the main casting or the trap itself, but you can only go so far with this type of adjustment. Schneider sells an expensive kit for checking this out.

As for the lens being on center or not I think you will find it on the image center for the most part, not on the 35mm full frame center. Some lens mounts have excessive adjustments built in and can compensate for being off screen center and such, so that may go as far over as being on full frame center. The DP-70 is a good exmple of a lens mount done correctly. I've seen adaptor collars that have markings for 70mm, 35mm optical and 35mm mag only... all of which have differing image center lines.

Hope this helps or touches on what you were contemplating.

Mark @ CLACO

Posted by Thomas Jonsson (Member # 1902) on 08-07-2004, 12:06 PM:

Hi Mark
Thank you for the info. Very interesting you
actually did this with your VistaVision projector.
I think I´ve seen a picture of it here at Film-Tech.

Have you considered trying this with a 35mm projector
for regular theater use?

Thomas

Posted by Mark Gulbrandsen (Member # 72) on 08-07-2004, 03:03 PM:

I've considered it but its difficult to do with a curved gate. You can put a ground plate in place of the aperature but thats about it. You would really need to refrence it to the curved runner somehow since when that part is closed its the refrence point for the film. Straight gates would be pretty easy to do on a Century but doing a Simplex and others by actually adjusting the positioning of the trap would be tough to do due to the way its mounted on locating pins. One could shim just the rails on a straight ggate version though, but its a pretty time cnsuming thing to do even when I set up the mounting foot the way I did. All the adjustment screws interact and then you go to tighten it all down and it all goes out of whacl! But finally it comes in and can be near perfect.

Mark