Saturday, October 3, 2015

Keystone Visual Survey Telebinocular Achromatic Stereoscope

From all possible optical aberrations, chromatic aberration is perhaps the one that bothers me the most!  Thankfully, a good stereo Realist format viewer (like my favorite, the Realist red button viewer) with achromatic lenses shows no chromatic aberration (except for the Kodaslide II, see my blog here). To check for chromatic aberration, just look through the viewer with no slide and pay attention to the edges. Do you see orange and blue colors? This is chromatic aberration.

Stereoscopes (for vintage 7 inch wide stereoviews) show chromatic aberration because they use prismatic lenses. Prisms (essentially asymmetric lenses) bend the light more than symmetric lenses and different colors tend to be separated more, hence the aberration shows up more easily. But still, not many people notice this, maybe because they do not know what to look for. But you should be able to tell when you use a stereoscope with achromatic lenses.  One of them is the Keystone Visual Survey Telebinocular, used for eye testing in the early 1900s (maybe up to the 1960s or 70s?). These have different model numbers like this one, labeled 46C Ophthalmic Telebinocular:

I remember how I got my first Keystone Visual Survey stereoscope (pictured above). I recorded the story in a photo-3d posting: A friend from photo-3d offered to sell me this stereoscope for a little over $100, back in 2000 or earlier. I accepted but felt that I was doing him a favor and wasting my money because I already had some very nice stereoscopes at home. Perhaps I could sell it and break even, I thought. The stereoscope arrived; I opened the box and took a look. "Ugly duck" was my first thought. I decided to give it a try. Grabbed a few stereo views and took a look.

Wait!!! Just a minute!!! What is going on here??? There is something in the quality of the image that I have not seen before. Are my eyes being fooled? I ran to the basement and pulled out all my stereoscopes. I checked the same view with every stereoscope I owned, plus the Visual Survey. No, my eyes were not being fooled. The image through the VS looked better than anything else!!! No sign of chromatic aberration (I could see false colors in every other scope I tried). Sharper images. More details, especially at infinity. Easier for my eyes. I could be viewing for hours with no fatigue.

WOW! This baby is here to stay! I gave it a good TLC cleaning and have it in my basement ever since. Whenever I want to enjoy stereo views, this is the scope I use. Later, from discussions in photo-3d and input from Alan Lewis, I realized the secret behind the optical quality of this instrument: It has achromatic lenses! While every other scope has simple wedge lenses, this baby has achromatic lenses. I know from my slide viewer experience what difference achromatic lenses can make. The large size of the lenses and large FL makes this difference lens significant, but it is still there. And a trained eye can pick it up very easily.

Here is a comparison between the lenses of the Keystone VS Telebinocular, vs. a standard stereoscope:

The VS Telebinocular lenses are thicker (21mm vs. 7mm). If you take them out, it appears that these are halves of achromatic lenses. So, a standard stereoscope uses half of a standard one element lens, while an achromatic stereoscope uses half of an achromatic (two element) lens.

The bottom line: I sold all my other stereoscopes and kept this one.

But, it gets even better: Eventually, I bought a smaller stereoscope with achromatic lenses from Alan Lewis. It is called the “Achromat Lorgnette II”. It folds and can fit in my pocket. This is my preferred stereoscope, only because it is portable and I do not view stereoviews very often. Here it is:

A beautiful (made out of wood) portable achromatic stereoscope. I believe I paid around $300 for it in the early 2000s. Too bad it is not made any more.

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