Film-Tech Forum ARCHIVE: Anamorphic setup

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Posted by Antonio Marcheselli (Member # 373) on 09-21-2003, 06:30 AM:

Hi everyone

For the first time I tried to adjust few anamorphic lens with the tip found here. IT'S GREAT, I fixed few lenses that were slighty out of focus.

I have two questions: one of our screen has the Flat image a bit out of focus at the right. Obviously the scope image is more defocused at the right. The lamp seems to be centered on the screen. Can be the len's holder not aligned with the film gate?

On some screen I was not able to optimize the sharp of the image on all the surface of the screen. If I use the tip found here I have to average the result because if I make a perfect image at center the edge of the screen are too much out of focus and viceversa. In this situations Flat image seems perfect. Can I considered "normal" that biggest screens are not perfect with scope at the edges?

Bye!!
A

Posted by Steve Kraus (Member # 476) on 09-21-2003, 10:07 AM:

If, as you adjust focus you can see the image go into and out of focus left to right or right to left all the way across the picture (or top to bottom) as you turn the knob then there is some sort of mechanical alignment problem between lensholder and gate/trap. This may be more apparent with some lenses because of different depth of focus between lenses.

If the focus difference is between both sides and the middle then either there is a lens problem or the film is not sitting flat (or flat enough in curved gate situation) and it's a gate/trap problem.

Posted by Paul Mayer (Member # 355) on 09-21-2003, 12:44 PM:

Regarding your first question, one other thing to look at if your machine uses a motor and/or stop screws to hold the lens turret in the show position--check the alignment of the turret when its closed. On one machine I recently looked at (a Strong) someone had adjusted the turret stop screws all the way in, causing the turret to swing closed past its correct position. Couldn't get a good focus evenly across the screen. After re-setting the stop screws (by eye--didn't have my Align-o-Tron that day) and re-positioning the projector to re-center the image on the screen, focus was much better.

Regarding your second question, is this a short lens (big screen in a short theater)? If flat looks OK but 'scope does not--perhaps the back-up lens for 'scope does not have enough depth of focus for this application? Maybe try another back-up lens, a slower one or look at using stop-down rings (and live with less illumination--usually not a problem for 'scope if flat looks OK) in order to achieve good 'scope focus. Ideally you should be able to achieve even focus for 'scope if flat looked good on the same screen.

Posted by Antonio Marcheselli (Member # 373) on 09-21-2003, 05:03 PM:

Steve,

Yes, if I turn the focus knob I have perfect focus at right but not at left and viceversa. Seems that I have to register the mechanical stop of the lens' holder! (Vic5).

Paul
I'm talking og big screen in short theater. This is why I suppose as "normal" that at the very edge of the screen the imagescan be just a bit out of focus. However, to answer to your question, I'm talking about focus at center but not at borders. But not so much out of focus! Unfortunately our projection windows are not of best quality, perhaps they are guilty of this problem...

Bye!
A

Posted by Paul Mayer (Member # 355) on 09-21-2003, 06:46 PM:

Yeah, big screen in a short theater is an extreme condition, though not unsolvable. Isco and Schneider both make lenses specially designed for wide angle throws. Maybe you can try one out on a loan?

Port glass can certainly cause image quality issues, but I don't think even the worst port glass would cause a difference in focus across the screen. Perhaps you can pull the glass out just to check?

Ciao!

Posted by Antonio Marcheselli (Member # 373) on 09-21-2003, 06:54 PM:

Yes, I'll try because our lenses are already Schneider / isco!

I'll post the exact models

ByeA