Tri-Delta Prism Stereo Camera Adapter

The Tri-Delta Prism Stereo camera adapter is an image splitter for stereo photography. Intended for mounting like a lens filter, this device uses an equilateral prism and two first surface mirrors to divide a camera picture into right eye and left eye components. Originally intended for slide photography, the unit originally shipped with a slide viewer as well.

Lens Filter Adapter:

My Sony DSC-S50 digital camera apparently has a wider angle of view than the usual film camera, and I found that I needed to mount the lens of my camera as close as possible to the back of the prism to get the best use of the digital field of view. I did try using a couple of filter ring adapters to get my 37mm lens filter holder to work with the original "Series V" adapter (check B&H Photo). One could also have a custom adapter manufactured (S. K. Grimes machinists). Here is the original adapter on my Tri-Delta below.

I thought I could do a reasonable job myself, so I fashioned an adapter to replace the original "Series V" adapter on the Tri-Delta. I bought a 37mm-58mm (lens to filter) adapter, and removed the outer 58mm threaded ring with a Dremel tool, then epoxied a matching ring of 1/16" printed circuit board material (FR4) to the remaining aluminum ring. The center hole is squared off fore and aft as light baffles to match the body of the Tri-Delta. The drilled holes on the adapter are not exacty in the mid line of the adapter strangely enough. I spent some time deciding exactly where to put the holes so that when I snug the adapter onto the camera lens body, the Tri-Delta just squares up with my camera. I also found that I needed to shave off about half of the thickness of the FR4 on the left side for my camera because the lens of the camera is not aligned with the filter mount on the case of the camera(!). I used a huge permanent felt tip marker to "paint" the fiberglass FR4 material black.

Below is the new adapter mounted on the Tri-Delta with new 4-40 flat head machine screws from my local hardware store. I used the original nuts on the inside.


My first step in this project actually was to remove the mirrors and send them off for recoating. These are first surface mirrors, and had maybe 2/3 of the original aluminum coating still on the mirrors. I was not too impressed with the foggy and dim appearance of the first few pics I took, so I sent them to Majestic Optical Coatings and for $75 had the mirrors recoated with protected aluminum. The prism is in good shape, which is fortunate, because the prism is glued in place. Below is a stereo self portrait of the Tri-Delta with newly recoated mirrors and my digital camera. This is the orientation in the original instructions for the Tri-Delta, but I find that holding it upside down allows me to see my camera LCD viewfinder much more easily.


Once had the mirrors back in the Tri-Delta, I noticed that the left and right images were not well aligned and this wasted data in the final superimposed images, so I spent an hour fiddling with the tiny set screws on the back of the body to align the mirrors. The screws were kinda frozen initially, but yielded finally to strong firm pressure. Once I had the physics of the alignment in mind (right screw moves left image up/down, both screws together for toe in/out), it went pretty easily. Inside the Tri-Delta body, I could see the metal strip springs that hold the mirrors--if the springs were completely compressed, screwing in the set screw further could break the mirror, so I would back out the screw a little and work on the other side. I could have probably saved myself some time by just eyeballing the springs and adjusting the setscrews to a middle compression of each spring. You can see the hole for the right set screw (one on each side) on the back of the Tri-Delta body below. You can see also the viewfinder lens below and to the center.

Flash Adapter:

Since the camera body points perpendicular to the Tri-Delta, the built-in flash will not work without some additional modifications. Since my camera does not have an external flash connector, I set about redirecting the existing internal flash. Right next to the flash on my camera is a photocell for the flash unit, and it needs to be redirected too. I went by a local craft shop, (Ben Franklin), and picked up a 4x4" sheet of second surface mirror for $1. I could have made a mosaic of smaller pieces work (they had mirror pieces in several sizes), but I wanted to try my hand at cutting glass also. So I picked up a hand-held glass cutter for $4 at my local hardware store (the craft shop was out of the glass cutters). I read on the net about cutting class, put a little oil on the cutting head, then used the cutter to scribe a line across the front surface of the mirror about 2cm from one edge. I turned the mirror over, and used the other bulbous end of the cutter to gently tap along the cut. With one or two taps, I had two pieces of mirror with a good cut. Ecstatic, I made the other cut, resulting in a 2x4 cm mirror. I used a regular hardened steel file lightly to take off the sharp edges. Using some hardwood, I fashioned a holder, glued the holder to the back of the mirror, and used some 1/2" double sided foam tape to stick the contraption to the front of my camera. The tape did not stick well to the wood, so I hardened it up on the bottom with some thin cyanoacrylate glue (from my local hobby shop), and I used my favorite mega permanent marker pen to paint the wood black. I did a few test exposures in a dim room of a blank wall to test the flash distribution, and ended up gluing a shim on the thing to re-adjust the flash angle. Next test . . . perfection. Well, almost. The mirrors and prism lose some photons, and the flash really seems anemic with standard settings. I can make it passable by bumping up the flash intensity and the exposure for fill-flash photos.


Below is a raw photo from my camera, reduced in size, to show the arrangement of the left and right images, and to show the midline bar from the prism and mirror edges. Note also the wedges in the midline at the top and bottom. Even with the camera lens only a millimeter or two away from the prism, my camera still has a wider angle of view (in the usual 4:3 width to height ratio mode) top to bottom than the Tri-Delta adapter can offer. I can remove this easily enough in the lab (with Photoshop Elements) or by zooming slightly. You may be able to see also that the middle bar is not quite parallel to the sides of the image--I goofed lining up the Tri-Delta with my camera. The bar is also not quite in the middle of the picture--I need to shave just a little more off the left side of the new adapter. In the final image conversion, I ended up rotating one of the images very slightly for perfect stereo, and I'm not sure yet if this is because the camera and Tri-Delta alignment was off or if the mirrors are off. In any case, the data is looking pretty classy at this point.

I took the non-lossy compression TIFF mode 1600x1200 24bit version of this photo, converted it, and enhanced it a little, to produce this final image below. The image below is reduced in size quite a bit to save web site storage space.


By the time this system is up and running, I've spent a fair bit of money (the Tri-Delta is considered a collector's item) and time, probably as much as I would have spent on an additional digital camera for a twin camera rig. But every stereo system has its discipline with advantages and limitations of its medium. The Tri-Delta has NO shutter sync, flash sync, or white balance issues at all, all potentially thorny issues with a twin rig. I couldn't resist taking a final "cha-cha" stereo pair of the Tri-Delta too. You can see the two mirrors, the prism, the viewfinder, and you can just make out the steel leaf springs along the bottom of the mirrors that hold the mirrors in alignment.

Good luck,
Rob Crockett
Copyright 2003, 2008. All Rights Reserved.

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