Film-Tech Forum ARCHIVE: CS lens holder trouble

This is topic CS lens holder trouble in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Patrick de Groot (Member # 858) on 03-10-2003, 04:54 PM:

I'm having some trouble to get a good sharp picture with a CS lens (without turret). I've got a base lens which goes into a collar. That collar (metal hollow ring) goes into the lensholder. The CS attachment can be screwed on top of the baselens.

So at which distance should the baselense be of the filmgate (in combination with the focus knob)? I've heard I should first have a sharp picture with the baselens and then screw the CS attachment back on it and adjust its focus distance ring. Is this true?

Posted by Steve Kraus (Member # 476) on 03-10-2003, 09:07 PM:

No. Mate the two lenses securely then install it in the lens holder with the orientation properly horizontal. It's best to run a test loop like SMPTE 35PA/RP40 and make sure that all the horizontal and vertical lines are at 90° angles...this should result a properly horizontal image but you never know...your projector may not be level or the gate and trap alignment may be off.

You may need to experiment a bit to find where the lens is going to be in focus and how far into the collar to place it so that the collar ends up where you want it in the projector's lens mount. Generally you want it to stick out a bit in front so you can put a ring there which will allow you to put the lens in and have it be in focus just as was left from the other lens.

Place the lens+collar in the lens mount and move it until the horizontal lines (representing vertical information) are at maximum sharpness. Clamp the lens in position and adjust the projector focus to the absolute peak of sharpness, again looking ONLY at the horizontal lines. (If your projector focus has already been adjusted for other lenses and you do not wish to readjust the rings on those lenses you will need to focus entirely by moving the lens+collar around and then clamping down.)

Now on the anamorphic attachment release the clamp ring at the front or whatever sort of set screw or locking device it has, and adjust the focus on this lens only to maximize focus of the vertical lines (representing horizontal information). When you get the best focus on the smallest test chart elements lock the adjustment ring in position. You should never need to touch it again. All future focus adjustments should be done with the regular focus knob that moves the entire lens mount. Note that focus on ordinary film may differ a bit than what you set with the test chart but make the correction with the regular focus knob and leave the anamorphic unit alone.

When done you can put the alignment ring on the front of the collar, hard against the lens mount barrel of the projector, with the alignment pin in the slot, so you can change lenses and have them each be in good focus.

Posted by Patrick de Groot (Member # 858) on 03-11-2003, 02:41 AM:

Thanks for your clear answer!