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Re: [OM] Polaroid NPC backs (was: Olympus Speed-Magny)

Subject: Re: [OM] Polaroid NPC backs (was: Olympus Speed-Magny)
From: Frank van Lindert <lindertv@xxxxxxx>
Date: Fri, 04 Sep 1998 09:38:22 GMT
On Fri, 04 Sep 1998 04:52:25 -0400, Mark <om4ti@xxxxxxxxxxxxxxxx>
wrote:

>Frank,
>   I just pulled my NPC back out again. The transmission block is not
>plexiglass, but it does not look like it is made up of traditional
>plastic fibers. The block is semi-free floating mounted on a pair of
>pins. The block has considerable heft to it. To the naked eye it has the
>look of ground glass. Under my loupe it appears to be made up of hexagon
>shaped things placed in contact with each other.

That is exactly what I called optic fibers... They cannot take light
from their neighbours, so the small hexagonal parts of the image
coming in at one end of each 'fiber' are only transposed to the other
end, where the Pol.film is. As I see it, really the only reason of
doing it this way is to shift the focal plane toward the Pol.film with
as little image distortion as possible. 
BTW, I do have a part of an Olympus endoscope, and the optic fibers in
that have a hexagonal crossection too.


>But they do look and
>feel like glass. There is no curvature whatsoever. The sides have the
>look of unpollished rough glass. The block is in contact with the
>Polaroid film at the back end and extends in to the plane of the 35mm
>film path. Perhaps this is a change between the NPC Pro Back and Pro
>Back II. Mine is a Pro Back II. 
>
>   The reason for the tripod adapter has nothing to do with depth (about
>1"), it is height. A Polaroid series 100 pack is about 4" tall. The
>Olympus back must be at the top of the pack film or you would not be
>able to utilize the canera's viewfinder. An Olympus back is only 2"
>tall. This leaves over 2" of NPC back to hang at a level below the
>camera body's floor plate. The extender is a simple cylinder of steel
>that places the tripod socket below the bottom edge of the NPC back. It
>is possible that some tripods can function with 2"+ of camera hanging
>below the mounting point, a ball head can. But not all tripods will
>allow this much overhang, some will not allow anything below the
>mounting point if the screw is far enough forward on the tripods plate.  

Yes, Mark, you are quite right here. The position of the back,
necessary to leave the eyepiece free, is also the reason that only two
(and not 4) images fit on the polaroid sheets. Another waste... ;-)

Frank van Lindert.

  
>
>   I do not have a scale handy, but my empty back is only about one half
>pound. Then you add the weight of a series 100 pack of film.
>
>                                Mark
>
>Frank van Lindert wrote:
>> 
>> Are you really sure about that, Mark? I once carefully examined a back
>> like these (from NPC) which was made for Nikon, and I also used it for
>> one afternoon. From my examination and from the poor image quality I
>> concluded that the 'block of plexiglass' was indeed made up out of
>> optic fibers, glued or welded together.
>> 
>> How could you reach the desired shift of the focal plane (from the
>> original 35mm film distance to the longer Polaroid film distance) by
>> inserting a block of glass? (BTW, if you could, why use scratchable
>> plexiglass?)
>> 
>> As I see it, the only other way you could achieve your goal would be
>> replacing the original film by a fine ground matte, and projecting its
>> image on the Polaroid film. Maybe you could even use the air image and
>> leave the matte out altogether.
>> But in that case you would need one or more lenses in the Polaroid
>> back... Does your back perhaps have a lens built in? This would also
>> explain your remark on the tripod adapter needed. The backs I have
>> seen weren't that deep... but they were certainly heavy.
>> 
>> Frank.


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