Re: Achromatic StereoScope

[P3D Dr. George A. Themelis <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx> To: photo-3d@xxxxxxxxxxxxxxx]

>> How do you go about using achromats? You are not cutting them in half, >> are you?

> Yes , I am cutting them in half...
> Using achromats in a Holmes viewer is purely a matter of cost. One knowledgeable
> person on the history of scopes speculated that achromats wouldn't help a Holmes
> viewer because the main benefit of achromats shows up at shorter focal lengths.
> I don't know if this is true, but the use of achromats in a Holmes viewer makes
> for a better viewer from my observation. 

This discussion made me curious enough to open and take apart the lenses in my Keystone 46B Visual Survey Telebinocular. This stereoscope was discussed earlier
in photo-3d. When I first got this stereoscope I was thrilled by the quality of the image. Unlike every other stereoscope that I have seen, this one has no chromatic aberration and delivers pin-sharp images. I did a side-by-side comparison and concluded that the images are sharper, there is no chromatic aberration, you can see far away and enjoy the stereo image more than any other viewer. A couple of lucky owners of this rather rare sterescope agreed that this is the best they have.

I am holding one lens in my hands right now. Sure enough, it is made up from two elements. I do not know if it is an achromat cut in half or a specially designed two-element lens. The size is 33 mm in length and 22 mm in center thickness. As I said, this lens delivers the best image I've seen in a stereoscope. Alan, I can send you the lens to examine if you want... 

The knowledgeable person on the history of scopes who speculated that achromats would not help a Holmes viewer because of the longer focal length, has a point, BUT for those of us who can appreciate optical quality when they sees it, going from single to double element lenses MAKES ALL THE DIFFERENCE IN THE WORLD! 

I would personally pay the extra difference and get a viewer with better lenses. I am sick and tired of chromatic aberration! I know I am not alone. Once you get used to seeing pin-sharp and color-free images you don't want to go back to single lenses.

Alan Lewis writes:

> I am very pleased with the results from the prototype. The achromats really > do a better job than any PCX or DCX single element lens. The views appear to > have more depth and much sharper edge definition. 

Yes! I agree with this statement. The views do appear to have more depth. That's because you can see more detail far away. 

> But is the stereo public willing to pay the extra cost? A few will I'm sure, > so achromats will probably be a rare item in Holmes viewers. 

There are people who use single element lenses to view stereo slides and some of them claim that they do not see much difference. IMO, the difference is HUGE. It makes no sense to spend thousands of dollars in camera equipment and not want to invest money in a good viewer, a viewer that will help you enjoy your photographs. The same way, a collector who spends thousands of dollars to build his collection should give no second thought into buying the best viewer he can. When I first got the Keystone "achromatic" viewer I spent many hours going over and looking at my collection of views... and I had a great time! 

Alan, I think you are in the right track and you are offering a great service for those who want the best. Please count me in for one of those babies... The problem with the Keystone viewer is that it is not portable... It has been called a projector from those who do not know better... Thank goodness, my wife did not object when I declared that it will become a permanent fixture in our house... But I still need a portable viewer to carry with me. 

George Themelis


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