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. 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.
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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