Film-Tech Forum ARCHIVE: Anamorphic Centers

This is topic Anamorphic Centers in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

To visit this topic, use this URL:
https://ft-forum.com/ft/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=1;t=006977

Posted by Neil Fish (Member # 698) on 02-25-2005, 03:52 PM:

I recently did some image correction work (plates, masking, etc...) on single lens Simplex 35s, and had lots of difficulty getting flat and scope to match centers, or more importantly to get .825 centered on the maskings. These were Schneider Super series anamorphics, and I discovered by rotating this lens 180 degrees, I could match the scope center with flat. The flat lens (also a Schneider) kept its' center regardless of orientation. This activity was moving the image .005- .010. Not so with Isco lenses that were purchased and installed where required to correct images. Anyone have similar experience?

Posted by Mark Gulbrandsen (Member # 72) on 02-25-2005, 07:50 PM:

Yes, on quite a regular basis over the years. Both the lens mount and lens bushings should be checked to be sure they are concentric and not distorted. Lens mounts do stretch over the years, especially the old Simplex's as thay are basically just made from pot metal(die castings). I've had so much trouble with lens bushings over the years that I now check each one before it leaves the shop. Many are incorrectly machined off axis and its common to find the older Simplex ones distorted(again pot metal die castings!). Its also imperative that all adaptors be the same I.D. and O.D. or problems in centering will arise from any that are not exactly the same in those dimensions. You can't mix the old with the new or Simplex with Century, etc and still expect to be aligned in all formats.

Lenses can also have their optical centers off, this is common and more typical on anamorphs because the 2X expansion tends to magnify any amount the lens is off optical center one way or the other.

The other method which is a bit extreme and expensive is to use all offset correctors instead of the standard 4" adaptors. This way every format can be adjusted to exact screen center.

All in all it gets somewhat complicated to discern exactly what the real problem is but careful testing of components definately weeds out any bad parts before they can get into a booth.

Mark @ CLACO

Posted by Sam D. Chavez (Member # 1841) on 02-25-2005, 09:06 PM:

Assuming you are not using turrets,

There is always a bit of out of roundness somewhere and you can use this to your advantage. It's simply a matter of identifying and getting the scope and the flat to line up on a given vertical line.

It's even more difficult to get two projectors lined up perfectly on all lenses without using undersized plates.

Posted by Michael Schaffer (Member # 1204) on 02-25-2005, 09:39 PM:

Why would you need undersized plates to line them up?

Posted by Neil Fish (Member # 698) on 03-01-2005, 02:16 PM:

Sorry for the couple days absence here- out in the field.

The eccentric or worn lens collars do not apply here as they were left in their original orientation while the anamorphic was rotated within, so the shift in center was definitely in the lens (in the anamorphic only- the flat and scope primary were OK). It did occur to me to rotate the collar to take advantage of any tolerance differences, but I discovered the lens part first. It does not seem to be an issue with something loose or defective in the lens as the focus is even side to side, and within SMPTE specs. It is sort of academic at this point, I just wondered if this was common.

Hey Sam, I haven't spoken with you since your Dolby days.

Posted by Brad Miller (Member # 2) on 03-01-2005, 07:44 PM:

I've had to cheat on single lens projectors many times before by placing a strip of tape on one side of the lens before inserting it into the lens collar. Often I am only missing the dead centerline by a square on PA35, but I'm too picky to let that slide. It works quite well. Give it a shot.

Posted by Neil Fish (Member # 698) on 03-02-2005, 02:23 PM:

Thanks Brad- Once I read your suggestion, it occurred to me I could have used shim stock of various thicknesses to get a more precise adjustment, and it would not compress over time or temperature.

Posted by Mark Gulbrandsen (Member # 72) on 03-02-2005, 11:14 PM:

quote: Neil Fish
Thanks Brad- Once I read your suggestion, it occurred to me I could have used shim stock of various thicknesses to get a more precise adjustment, and it would not compress over time or temperature.

I use Brass shim stock. It can be cut with scissors.

Mark @ CLACO