Re: Stereo Card Specifications

[bercov@xxxxxxxxxx (John Bercovitz)]

When John Vala asked about specifications, I passed the question on to
Alan Lewis, formerly of this list, who makes fine stereoscopes.  His
answer agrees well with Ed Comer's answer.
 
Tim Shih asked about a mounting guide for stereo cards.  I don't
know of one but I'll tell you what I did to make one for medium format.
I just drew the guide in a CAD program and printed it in a laser writer
on vuegraph material.  It's very accurate and it works great.
 
Well, enough of that; here's what Alan said:
 
======================================================================
 
> From: LDAEnt@xxxxxxx
> Date: Sun, 18 Feb 1996 16:28:41 -0500
> To: bercov@xxxxxxxxxx
> Subject: Re: Stereo Card Specifications
 
I found the information quicker than I thought.  That's where organization
pays off!  I credit Craig Daniels (Stereo Type) with much of the viewcard
infinity information.
 
> John Vala, a long-time member of the list asked this morning what the
> standard specifications of a stereo card are.
 
For card size: 7" wide is standard, and the height was 3 1/2" before around
1880, and two other sizes of cabinet cards at 4" and 4 1/2" height were
introduced.  The 3 1/2 is the standard.  A modern Q-VU mount is 3 5/8" tall.
 
> What is the infinity separation, the aperture separation, and the focal length
 
Infinity separation has no set standard, just ranges.  Some cards have a
fixed format (no gap between the photo pairs) and others have fixed infinity
separation (these have a gap between the photo's).  It seems that the antique
cards used 3 3/8" maximum inf. separation, but this would change depending on
the company doing the work.  The typical inf. separation was 3 1/8 to 3
3/16".  Modern standards are 3 3/16 to 3 1/4" inf. separation for scenic
shots.  Closeups are less, usually down to a minimum of 2 7/8".   For
closeups you would have to make your photo's narrower.
 
An interesting side topic that I never realized until I started making
viewers;  the separation of the lens pairs in the scope affects the inf.
separation too.  All scopes are not identical for lens separation.  The lens
spacing in the scope sets what you will perceive as infinity (zero divergence
viewing).  If the scope is set at 3 1/2" optical spacing (when checked at
infinity) then a card with this spacing between inf. images will look
natural.  But scope lenses are not focused at infinity when used.  Therefore
the inf. separation is almost always less than this to make for a very slight
convergence of the eyes at infinity, and is usually more comfortable.  And
many scopes are a little more than 3 1/2" spacing.  So you can see that we
are not standing on firm ground here relative to standards. Luckily the human
brain can ignore these minor differences, and perceive the view as being real
anyway.
 
The gap between photo pairs can either be zero (where you would move your
inf. separation), or about 3/16" (where you would hold a constant inf.
separation).  You can adjust the gap or photo size within the boundaries of
the card in whatever combination you wish.  Certain combo's will be more
comfortable than others.   Whatever you try, just keep the inf. separation at
3 1/4" or less for comfort.
 
The focal length of a typical Holmes-Bates viewer is 5 diopter.
 
> were these views contact printed or enlarged and with what focal length lenses
> were they taken?
 
Can't find any good information on these questions.  I believe that since the
early photos were glass plate type then contact printing would be done.  In
mass production in the 1800's they used the sun for exposure, so I would
imagine that contact printing was commonly done.
 
I have read mention of using 5 or 6" lenses in stereo view cameras (around
1900).
 
Hope this helps.
 
Alan Lewis


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