[photo-3d] Stereopticon 707 (was: Give the gift of stereo)

[Abram Klooswyk <abram.klooswyk@xxxxxx>]

There seems to be disagreement in the answers to Linda 
Nygren's question (Jun 24, 2000):
>Is anyone familiar with their [Taylor-Merchant] folding print 
>viewer "Stereopticon 707"?

Gabriel Jacob (Jun 24, 2000):
>(...) it works fine with full size Holmes cards. For the 
>price they are pretty good!  (...)

Ron Beck  (Jun 26, 2000):
>It's flimsy and doesn't stay "unfolded" very well.  IMHO, 
>a hand-held lorgnette viewer would be more useful.

I have several of them, as they were sold with many types of 
stereobooks. 
There are two different designs of the Stereopticon 707. 
Their general design is similar, distance from lens to book of 
standing viewer is 135 mm for both.

But the older ones, produced since the late 1960s, were of 
cardboard (black) and stand rather firm when "unfolded".
The new ones, produced since about 10 years (?, not sure) are 
made of thinner material, probably some plastic, which is a 
little too easily deformed and stands less firm.

Other differences: 
Lens separation: old 63 mm, new 69.5 mm
Lens focal length: old about 165 mm, new about 150 mm.
With their identical lens-to-book distance, the difference in 
focal length means that their virtual image distance is 
different. For the old type it is about 75 cm, for the new 
about 135 cm.

There are more things to worry about. In a thread on 
"Inexpensive viewers?" Alan Lewis wrote in PHOTO-3D Digest 
1115, 12 Jan 1996: 
"There are two basic decisions to make before looking for a 
viewer.  If your prints are going to be bigger than 2 1/2" 
square then you will need a viewer with prismatic lenses.  If 
your prints are smaller than 2 1/2" sq. then you can use a 
straight lensed viewer (one with conventional lenses)."

The lens separation of about 70 mm in the new type 
Stereopticon 707 can have some "prismatic effect", it can 
be used for prints having an infinity separation of slightly 
over 70 mm. But most vintage views have separations of some 
80 to 85 mm (some however less, like most tissues). 

Viewing a card with 85 mm separation with a viewer having 69 
mm separation means looking with a divergence of some seven 
degrees (at the far points). This is no problem for many of us 
who practise parallel free viewing (some even of the old 
views), but don't do this to novices. 

Gabriel Jacob wrote (same ref.): 
>... focal length is about 137mm.
This reminds of another Alan Lewis post, on "Holmes focal 
length", PHOTO-3D Digest 1334 14 May 1996:
"It sounds, from your values, that you may be measuring the 
distance from the card to the lens after you focus it with 
your eyes (through the lens).  You will get these distances 
with this method.  But the focal length of lenses is measured 
by projecting a point at infinity through the lenses onto a 
card. When checked this way the sliding stage will be way at 
the end of the tongue, almost falling off."
A similar answer applies to Gabriel's value. Lens-to-book 
distance is not the focal length.

Note that all of the above was not about the Taylor-
Merchant 101 and 102 folding stereoviewers.

Linda: >No comments on the name are necessary. 
Indeed, they shouldn't have done this, but after so many years 
we have to live with it. (A funny thing is that after over 30 
years the company still prints "Pats pending" on it. Must have 
dropped to the floor long ago).

Abram Klooswyk