The lens from the Peterson 6x6 Viewer would work very well as an eyepiece for a medium format 3D viewer. I have a pair of them and mounted them in a piece of stiff cardboard for testing purposes. I spaced them appropriately for the Rocky Mountain medium format slide mounts. The view through these lenses is very good. They perform as claimed in the web page advertisement on the Marine Camera homepage. I experienced no color fringing nor blurriness in my views, even at the extreme corners of the slides. Everything was nice and sharp.
The lens specifications that the company gave me were 35mm diameter by 75mm focal length. Their ad states the lenses are 34mm diameter. I measured the actual lens with my dial calipers and got a diameter of 35.9mm but the mounting ring has a 32.4mm ID at the smallest point. Interestingly, when the lenses arrived, they were mounted backwards in the rings. When I first looked through them, I thought they were no better than my $6.00 dcx lenses, but when I discovered that they were in backwards, and turned them around, they worked like a charm. The fellow at the store said he normally turns them the right way before he sends them out but must have missed these. Be sure that the flatter side is towards your eye when you judge them.
The mounting ring is molded in black plastic. It is a cylinder with a wall about 1/16" thick. 2/3 of the way down the barrel is a small flange that keeps the lens from slipping farther into the barrel. it is this flange that has the 32.4mm id. it is about 1/16 of an inch thick too. the lenses fit snugly in the barrel but are loose enough that you can get them out easily. there are two tabs spaced at 180 degrees at the bottom of the mounting ring. these allow the ring to snap into the mounting plate. they are very similar to the method described by John Bercovitz in a recent post. the ring is completely invisible when looking through the lenses and the tabs are only just noticeable. if they are placed at the top and bottom or on the sides (not the corners) they will cause no problems whatsoever. I only noticed them because I was looking for them for this review.
To test the focal length, I held the cardboard lens plate parallel with one of my slides over a light table. On the side, I held both the cardboard and a metric ruler. I moved the lenses up and down till I found the best focus. When I looked at the ruler, it looked to be about 85mm from the slide surface to the center of the lens. I did this without benefit of my glasses. When I put them on, the best focus looked to be the 75mm that I was told. Without more accurate measurements I can't say whether the 15mm depth of field is accurate, but it looks like it is about that.
The entire viewer available from Marine Camera costs about $110 but the lenses are available separately and are priced at $39.95 each plus shipping and handling. I was quoted $34.95 for volume purchases of 50 or more. The lens as I received it also came with a mounting ring. You will also need to get some sort of eyepiece to use these lenses. When I got them, I did not realize that they slip out the front of the ring. Luckily, I noticed some looseness and took one of the lenses out purposely before it fell out. It was then that I noticed that the rounder side of the lens was mounted towards my eye. Marine Camera also has the rubber eyepiece available for an additional $4.95 which is cheaper than we can have one manufactured for. Another useful accessory is an eyepiece cap for $4.95. You can purchase any or all of these.
I had no trouble seeing an entire slide without my glasses. There was even a little to spare. With the glasses touching the front of the lens (I was doing this very gently), only the very tips of the corners were touching. A little farther back, portions of the slide start to disappear. All tests were done with the lenses in their mounting rings.
