There are many stereo Realist format slide viewers from the 1950s. The best ones have achromatic lenses. Some names: Stereo Realist, Kodaslide, Revere, Wollensak, TDC. Realist and Kodak made more that one model. For example, the Realist red button has achromatic lenses while the Realist white button has single element (glass lenses). The Kodaslide I has plastic lenses, the Kodaslide II has coated glass achromatic lenses (day and night difference).
Among these viewers, the question usually boils down this: Which viewer is better, the Realist red button viewer or the Kodaslide II viewer? This question would have raised a war in the 1990s (same as the question "which stereo camera is better, the Kodak or the Realist?") but I don't think anyone cares as much any more. But, in any case, I feel obligated to state my opinion.
I have seen, tested, repaired, all kinds of vintage viewers. The quality of the lenses is one factor, how well the viewer handles or works is another, when it comes to finding a personal favorite.
I have been very happy with Realist red button viewers. They have excellent lenses, metal rack and pinion and focusing. But they are fragile and they tend to break if your drop them. There is some variation in the optics. Some viewers show haze (loss of contrast) in various degrees, due to deterioration of the glue that holds the two elements together. If you find one with crystal clear lenses, then it is a joy to use and, in my opinion, the best vintage viewer ever made. This viewer can also be widened for 8p so you can view any image size you like. I offer this service, see: http://www.realistrepair.com/RBRepair.htm
[By the way, I repair all kinds of viewers so I am not trying to promote this viewer for this reason]
The Kodaslide II viewer has larger and coated "achromatic" lenses, which are, I am sorry to say this, some of the worse achromatic lenses ever put into a viewer. Just look through this viewer with a black and white slide or no slide just an empty mount, or nothing. You will notice: 1) Distortion (straight lines are bending, this is seen in all viewers but to a larger degree in this viewer), 2) Chromatic aberration (you will see colors at the edges, something that you normally do not see in achromatic lenses). Center sharpness is very good. The problems show at the edges and you can see them if you look at an empty slide mount (the image just confuses things so just look at an empty mount).
Now, I know that many people love this viewer and are not bothered (or cannot even see them) by these issues. Some claim that they do not exist. The fact is, these problems exist in every Kodaslide II viewer. I have tested hundreds of viewers and I can assure you that they are all the same, there is no individual variation. That's fine. The viewer has a nice friendly shape, the nose fits well, the larger size of the lenses is a plus, and used in AC it has a wonderful warm light. But the focusing sucks and the interocular adjustment and lens holders are all plastic and tend to break. Also, it cannot be widened as easily as the Realist viewer and certainly no wider than 7p (without a lot of difficulties).
In the end, it is all subjective. I had a good friend who loved the Brumberger viewer because it is simple to use, despite the fact that it has single element lenses. Another friend cannot tell the difference in quality between a Kodaslide I and II (to put it mildly, you have to be blind not to see the difference between these two viewers - the Kodaslide II has much better lenses, or, another way to put it, the Kodaslide I must have the worse lenses ever put into a stereo viewer).
The bottom line is that the best viewer is the one that you like the best. For me, this is the Realist red button viewer widened for 8p. Here is my favorite viewer:
Apertures widened to 8p, interior painted white, fitted with a halogen bulb. Through the years I acquired almost every stereo slide viewer ever made, including some very expensive custom-made viewers (by Ekeren and deWijs) but, in the end, this is the only viewer I kept.
For a different take on this, read this “Classic Stereo Viewer Shoot-out Results”: https://groups.yahoo.com/neo/groups/photo-3d/conversations/messages/103374 and feel free to form your own opinion.
I stand by my opinion:
The Realist red button viewer rules :)
The Realist red button viewer rules :)