Re: King of viewers & Achromat StereoScope Message-ID: <320CD4BC.2D21@xxxxxxxxxxxxx>

[P3D Alan Lewis <dlewis@xxxxxxxxxxxx> To: photo-3d@xxxxxxxxxxxxxxx]

P3D Dr. George A. Themelis wrote:

>
> I was under the impression that the reason that achromats are not used in > stereoscopes (as they have been used extensively in stereo slide viewers) is > that,
> unlike the spacing of Realist format chips, the spacing of the stereo viewers in > a standard stereo view is wider than the spacing of the eyes. Some kind of > prismatic lens is needed in the Holmes stereoscope to diverge the light path. >
> How do you go about using achromats? You are not cutting them in half, are you? > Do you just use large lenses? I have been able to view stereo cards using > achromats of large diameter. The large focal length makes a slight divergence > easy to achieve.
>
> George Themelis

Yes , I am cutting them in half. This is how Holmes format lenses are usually made whether they are single element or achromat. When cut in half and transposed the lenses form the diverging prism. The most stressful part of this project was cutting the achromats in half. I didn't know what to expect with the cutting operation, but it went very well, so now I'm ready to cut any glass lens and trim it. The antique lenses (single element) were hand split. You can't do this with achromats for obvious reasons. 

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.
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. Besides, what manufacturer can afford the cost of custom achromats of the dia. needed? 

Alan Lewis
dlewis@xxxxxxxxxxxxx


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