[photo-3d] Notes on Tri-Delta Prism stereo adapter (LONG)

[Peter Davis <pd@xxxxxxxxxxxxx>]

I had spent several months trying to assemble the parts I need for a 
Tri-Delta Prism stereo adapter.  I managed to get an adapter (without the 
attachment rings) from eBay, and a viewer from eBay in a separate 
auction.  I got the rings from B&H, and even found an old railroad 
conductor's punch which serves pretty well for making the notch in the 
slide mount.  Since I've finally been able to shoot one roll of film 
(Kodachrome 64) and get the results back, I wanted to report on my impressions.

I'm always looking for an "easier" way to take high quality stereo 
slides.  This approach seemed very promising, since I could use my regular 
SLR, but not have the keystoning and vertical format restrictions of a 
regular splitter.  (The Tri-Delta apparently does have some keystoning, but 
since the images are rotated 90 degrees on film, the distortion is 
symmetric in the two shots, and is very slight anyway.)

I had also thought of using the Tri-Delta with a digital camera.  I don't 
have one yet, but I thought this would be a good way to do digital stereo 
without having to buy two cameras.  This would also eliminate some of the 
peculiarities of the Tri-Delta format, since I could easily split and 
rotate the two images in Photoshop.  However, finding a reasonably priced 
digital camera which has threaded filter rings is a problem, especially 
since the Tri-Delta has Series 5 threads.

Getting the attachment on the camera was the biggest problem.  I wound up 
buying a 52mm-Series 7 step-up ring, and a Series 7-Series 5 step-down 
ring.  The net effect is that the Tri-Delta fits nicely, but is probably 
about 1 1/2 mm further from the lens then was intended by the design.  This 
may have been a contributing factor in what seemed to be the biggest 
problem with the adapter ...

Vignetting!  There is visible vignetting in almost all my 
shots.  Unfortunately, I was not careful enough to record all my aperture 
settings, so it may be that this is only a problem at smaller 
apertures.  Still, it's something to be aware of.

The Tri-Delta turns the camera into a sort of periscope.  In other words, 
the attachment opens towards the bottom of the camera, so to take a picture 
of a person, you point the camera body up at the sky, so the Tri-Delta 
Prism (TDP ... I'm getting tired of typing this) is facing the subject.  To 
simplify this, I bought a small right-angle viewer (also on eBay) so I 
could see my composition while holding the camera in this bizarre 
position.  It turns out to be pretty non-trivial to figure out what the 
composition is with an SLR, since the view in the finder window is two 
images, rotated 90 degrees and positioned top-to-top.  Still, with a little 
practice, this seemed to be easy to deal with.

However, because of the unusual position, there's no way to get a 
camera-mounted flash to aim at the subject.  One could use it as a 
bounce-flash, I suppose.  Another possibility would be to get a camera 
bracket that allows the flash to be adjusted in that position.  The only 
one I saw was for something like $120, so I didn't experiment with this.

Finally, the viewer itself leaves much to be desired.  It's small and 
light, which is good, but it offers little magnification.  This means you 
end up viewing an image that's the size of half a film chip.  Also, by 
being small, the viewer is less "immersive" than other stereo viewers ... 
it doesn't block out any peripheral vision, etc.  Also, the diffusers on 
the viewer (I tried two of them) seemed to have a visible grain, which then 
made the slides look grainier than they are.  This was very 
disconcerting.  I will try to remount some of these pairs in the RMM 
horizontal half-frame mounts that Paul Talbot and Don Parks devised, so I 
can see them in a real viewer.

So, overall, the amazing thing is that the contraption produces stereo 
views at all.  It's so bizarre looking.  I still think it might be an 
interesting solution for digital stereo, but the drawbacks, plus the bulk 
of the camera with the TDP and right-angle viewer attached, will probably 
deter me from using it very much with regular film.

-pd

--------
                              Peter Davis
                Funny stuff at http://www.pfdstudio.com
                "The artwork formerly shown as prints."
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