Film-Tech Forum ARCHIVE: Lens Mount Alignment

This is topic Lens Mount Alignment in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Steve Kraus (Member # 476) on 10-23-2003, 09:21 PM:

X-L (or descendants), single lens lens mount. Different focal length lenses project to different center lines in both planes. I presume this is a symptom of the mount not being lined up properly with the gate.

What is the best way to do this? Hopefully there is a better way than trial and error adjustments. I had a goofy idea but I don't think it will work. My idea was to get a laser alignment tool like Greg's Align-O-Tron and put it in backwards to point at the screen, marking the laser dot with a Post-It or something. Then project, say, 35PA/RP40 and align the mount laterally so the X center of the image is where the red dot had been. (Yes, the position of the the laser tool, clamped in the lens mount, would be physically off center if the mount is off but that would be on the order of a mm or two--insignificant in terms of the red dot many feet away while on the projected image a mm or two would make a big difference.) This takes care of the X axis but not the Y. I had thought about a pinhole plate and center the circle of light on the red dot position but plates are not accurate and may not sit in the trap precisely. So what's the best way?

Posted by Mark Gulbrandsen (Member # 72) on 10-23-2003, 09:58 PM:

Steve, It could be that you have several 4" adaptors that are out of tolerance concentrically. FYI, the Simplex 4" adaptors as made by Strong are miles ahead of the old die cast Simplex adaptors. Put in a lens that goes off center and with the lens mount lock screw just loose enough try rotating the adaptor inside the barrell and watch the center of something like RP-40 while you turn the adaptor. It the RP-40moves then either the 4" adaptor is bad or the lens is just not seated correctly inside it.
Another note: No two lenses have the EXACT same center concentricity either.....but the differences are sometimes hard to detect visually just by running film on a screen.

Mark @ CLACO

Posted by Michael Schaffer (Member # 1204) on 10-23-2003, 10:26 PM:

quote:
I had a goofy idea but I don't think it will work.

I reverse and point the Align-O-Tron at the screen all the time. It does tell you reliably if the lens mounts are centred.

Posted by Dave Callaghan (Member # 1544) on 10-24-2003, 04:00 AM:

I wonder just how precise anyone can be with a Simplex single lens mount.

The image moves laterally depending on how much you tighten up the lens fastening screw.

I have also experienced the image moving vertically when the focus is adjusted.

I have also encountered lens mounts that could be characterized as being eccentric — a rotation of the lens mount rotates the entire image.

Given all the variables, trial and error might be the practical approach.

Posted by Kenneth Wuepper (Member # 1174) on 10-24-2003, 06:27 AM:

Our Cinemeccanica V 10s have built in eccentric lens carriers. They are marked with positions for correcting the center shift with Scope Magnetic and also for 70mm.

If you forget to rotate the lens carrier, the images do not register with the maskings downstairs. The booth is on the 4th floor and the stage is 4 steps above grade.

Posted by Frank Angel (Member # 248) on 10-27-2003, 03:11 AM:

I just recently used the Align-O-Tron reversed for horizontal centering. I also discovered that pinhole plates are NOT accurate, I surmise that this is because there is no expectation that the plate will be used for alignment but will be cut to the need aspect ratio. Actual off-the-shelf ar plates are much more accurate -- if you stick in a 1.85:1 plate (not under-cut), you can expect that to be centered since it is designed to be used as-is and it can be use to measure the vertical position of the projector head.

As for the "pinhole," when it gets to the screen, it's far from a pinhole, but rather a basketball on the screen so you then have to start calculating the diamter of the ball....not a big deal, if you could trust it to be exactly centered.

The Align-O-Tron is a tool I don't know how I ever did without....thanks, Greg. Like FilmGuard, I just wish I had it had been in my tool box for the last 20 years!

Frank

Posted by Steve Kraus (Member # 476) on 10-27-2003, 11:22 AM:

As I said the left/right adjustment can be aligned to the laser dot using 35PA/RP40 as the reference but I haven't really thought of a good way for for vertical since framing is an uncontrolled variable. Maybe Greg will want to ponder this and come up with a way to extend the tool's usefulness. I'd buy one right away if it would solve all these issues for me. [Cool]

Posted by Steve Guttag (Member # 268) on 10-27-2003, 11:49 AM:

For vertical reference on the Simplex...I keep a "golden" 2.35:1 precut plate that I use as a vertical reference. They key Steve, is to use a common reference and have all of the lenses follow it.

Frame your 35-PA or Schneider CLT test film and aim the laser to the film...adjust as necessary to bulls-eye to center. It actually works quite well.

Vertical is less tricky other than the simple rule that you must use a COMMON reference.

Most Kinoton projectors have their "blind" plates with the pin hole actually be dead center...sometimes they mis drill the aperture plate reference roll pin so it misses. If it is aligned then aim your laser through the hole in the plate...and then reflect its image back right though the hole again (from the lamphouse) and you will have a GREAT optical alignment.

Steve

Posted by Michael Schaffer (Member # 1204) on 10-27-2003, 04:59 PM:

Maybe we could convince somebody with precise machining equipment (like Greg) to make sets of true dead center pinhole plates for the various projector types.

Posted by Brad Miller (Member # 2) on 10-27-2003, 05:22 PM:

I just put the Align-O-Tron in the lens holder and a short piece of PA35 in the gate, then adjust the turret closure such that the red dot line aligns perfectly with the black square in the middle of the target film. For vertical alignment I have a plate that I feel is properly centered vertically and I align to that by sliding the plate slightly out so that the red line hits the edge. From that point it is easy to simply center the height. ...then I start aligning the lamphouse. It is amazing the difference the Align-O-Tron tool makes with regard to focus alone. [thumbsup]

Hey Greg, if you decide to get into the aperture plate business, please, please, please make dual Century aperture plates! Strong's "undercut" plates are a joke. Their flat opening is dangerously close to the horizontal limit line and their scope opening is too close to the vertical limit line. As a result, if you want a 1.33 plate, you have to cut open the flat side! Also, if you have a 2 projector setup with some keystoning, beware the plate from the factory may bleed into the masking on the opposite side. There is simply no reason for them to cut the plates like that. [Mad]

Posted by Gordon McLeod (Member # 33) on 10-27-2003, 05:23 PM:

A cautionary note most lens turret closuere will effect the angle relative to the gate not the offset distance

Posted by Steve Guttag (Member # 268) on 10-27-2003, 05:47 PM:

There are four angles to get right with respect to lens/aperture...pitch, yaw, vertical and horizontal...merely pointing the laser at a single target won't allow you to determine the difference between a horizontal misalignment versus a turret being parallel to the film plane.

The Schneider tool will ensure precise lens to aperture alignement. The laser will allow precise horizontal/vertical alignment all of the way back.

Steve

Posted by William Hooper (Member # 71) on 11-03-2003, 01:03 AM:

Along this line:
In the Simplex 35 the P-2577 Clamping Screw, Lens Holder Adjustment, is almost always found to be loose & spinning. What adjustment is it clamping?

Posted by Pete Lawrence (Member # 130) on 11-03-2003, 10:09 AM:

Bill,

It simply keeps the P-2553 clamp and P-2554 spring from falling out. The whole assembly just provides some friction on the focus knob shaft to keep your focus from wandering due to vibration or whatever. If your lucky you might have a little adjustment in the amount of friction, but I've only seen these screwed down finger tight.

While we are here, anyone have a good trick for eliminating the backlash in the focus knob on a PR-1014?

Posted by John Walsh (Member # 168) on 11-03-2003, 10:41 PM:

We've had this topic up before....

The issue I have is; what order do you align everything? Just aligning the lens to the aperture isn't enough... as Frank noted, I've seen apertures that were manufactured wrong. To do a complete job, I'd like to see several items included. For example, do you align the intermittent sprocket to the gate, the gate to the aperture, the aperture to the lens mount, etc.... Different projector makes probably require a different order of alignment ....