Film-Tech Forum ARCHIVE: Lens extension tubes

This is topic Lens extension tubes in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Peter Mork (Member # 1336) on 06-05-2004, 01:50 AM:

Something I've often wondered about - what's the optical effect of adding extension tubes between lens elements? Some projectors require them, where for instance a fat scope element blocks the lens from being inserted far enough to acheive focus; adding a 3-inch tube lets you do this. Other projectors don't need the tubes. I can't see a dime's worth of difference between the extended/unextended versions, but surely...?

50 words or less, go easy on the obscure jargon please. Thanks.

Posted by Dan Lyons (Member # 1454) on 06-05-2004, 02:46 AM:

quote:
Some projectors require them, where for instance a fat scope element blocks the lens from being inserted far enough to acheive focus;

Really?
what make of projector and how short a throw for that to happen? [evil]

Posted by Peter Mork (Member # 1336) on 06-05-2004, 10:44 AM:

Brenkert BX-100 (and I suspect other old Brenkert models).

Posted by Tim Reed (Member # 137) on 06-05-2004, 10:51 PM:

If you go very far with it (like ridiculously far... 8-10 inches or so), you'll start to notice the effect of extending the focal length of the backup lens (picture gets smaller on the screen). But it's very minute.

Posted by John Walsh (Member # 168) on 06-06-2004, 10:24 AM:

I asked Schneider about this. They said the two lenses (the prime and the anamorphic adapter) should be as close as possible. But, if you have to do what's necessary to make things work. Have you projected some test film (35PA) and checked for problems?

In a perfect world, I suppose the 'right' thing to do would be to purchase an integrated anamorphic, which does not flare out to a larger diameter, like regular anamorphics. Unless your problem is the -image- hits the edge of the projector head, like short lenses installed in a AA-II.

Posted by Gordon McLeod (Member # 33) on 06-06-2004, 03:43 PM:

With long focal length back ups I usually add enough extensions so that the image coming out the anamorph is occupying most of its front element because then ideally any glass imperfections would represent a smaller aprt of the image percentage

Posted by Paul G. Thompson (Member # 655) on 06-06-2004, 04:09 PM:

As long as there is no vignetting present, I don't think it is too big of a deal. I don't like to have them take up as much area as Gordon does, but if you have a good quality late model lens, Gordon's suggestion will work just fine. If you are using those old 40-year old lenses, forget it. Otherwise, there just might be much spherical and chromatic aberrations that could be easily detected by the eye.

Posted by Peter Mork (Member # 1336) on 06-07-2004, 01:28 AM:

Hmm, Tim, now that you mention it - when setting up a scope lens for my Century, I usually just screw the back element directly to the anamorphic. But last week I took the Brenkert lens, with ann extender in the middle, and used it in the Century - hello, what's this? My perfect aperture plate is now casting shadows. Your "shrinkage" comment would explain why.

Posted by Brad Miller (Member # 2) on 06-07-2004, 01:38 AM:

Watch what happens when you swap out a Strong lamphouse with a Christie. Due to the way the bulb is focused past the film plane, the aperture starts to show.

Posted by Gordon McLeod (Member # 33) on 06-07-2004, 09:16 AM:

Most people tend to forget that the lamphouse is part of the optical system and it can cause changes in image magnification, focus and apperture shadow and lens efficiency and sharpness