[photo-3d] Re: viewer optics. Brewster, Helmholtz, van Albada 1/2

[abram.klooswyk@xxxxxx]

There where several interesting comments in this thread, some quoting 
Ferwerda, but then all of a sudden Paul Talbot moved to Tech-3D :-). 
Interested non-subscribers to that list should look at John
Bercovitz' redrawing at Paul's site: 
http://www.rmm3d.com/3d.encyclopedia/mounting/ViewerLensSpacing.pdf
>The drawings are similar to those in Ferwerda's book, with some
>added commentary.

Another similar drawing, also referring to Ferwerda, has 
already been online for some time at Eddie Bowers' View-Master 
site, recently moved to dotcom:
http://www.vmresource.com/vviewer.htm

This all brings back memories of the famous I.S.U. founding 
Congress, Wageningen, the Netherlands, 1975. 
Koo Ferwerda had sent out a questionnaire on "ideal" stereo 
systems. In the Report B for the congress he wrote about the 
opinions on the interocular distance of the stereoscope: 
"Some (...) remark: 'it must be adjustable to the eye-base', 
and others: 'adjustable to differing homologous distances'. In 
my opinion the latter are right."

I had done a quick literature search on the subject and wrote 
a paper for the technical working group of the Congress, 
"Statements on the Stereoscope". A drawing as now redone by 
John Bercovitz was already shown in two German 
stereo books of 1910 and 1914, both probably based on an 
original drawing by Moritz von Rohr, employed by Zeiss and 
connoisseur of 19th century stereoscopy.

But already Hermann [von] Helmholtz had pointed out the facts 
in his famous 1867 handbook on physiological optics (in German). 
He made alterations to Brewster's lenses stereoscope and 
wrote:
"Because the distance of corresponding points in the usual 
photographic stereoscope images not always is the same as that 
of the eyes (...) should one, in order to achieve the most 
natural projection of the objects, have the possibility to 
adapt the instrument to every image." (Literally translation, 
not supposed to seem English.)

So adapting the separation of the lenses to the separations in 
the picture, not to the interpupillary. 

Helmholtz also wrote: 
"I am used to draw out the tubes first. until the photographic 
stereoscope image is in the focus of the lenses (...)".

Of course all the schemes of different interpupillary 
distances using the same separation of stereoscope lenses 
depend on the right focussing.
When people have to accommodate in looking at the pictures,  
they will set the viewer interocular to a separation which 
causes proportionally viewing convergence. This important 
point should not be neglected in these discussions.

Helmholtz added: "(...) the fusion of the two images will not 
be disturbed even if the head is inclined to one side." For 
indeed, what the schemes do not show is that also with 
vertical movements of the eye the same scheme of parallel rays 
coming out of the lenses is applicable.

In my paper I showed that in many stereo books the erroneous 
opinion of adapting the viewer to the interpupillary can be 
found, and the origin obviously was David Brewster's original 
paper on the lenticular stereoscope (1849, 1852).
He wrote: "The semilenses should be placed in a frame, so that 
their distance may be adjusted to different eyes." 
And again in his 1856 book on "The Stereoscope":  
"(...) slide in grooves so as to suit different persons whose 
eyes (...) are more or less distant."

Note that this had nothing to do with looking centrally or 
axially through the lenses, Brewster used lenses cut in half 
(semilenses), so _all_ viewing was extra-axial, as it is in 
Holmes stereoscopes till today. 
In his book Brewster has a chapter on difficulties in using 
the stereoscope, but no mentioning of the separation of 
corresponding points can be found. 

In next posting more on optical center versus non-axial viewing.

Abram Klooswyk


 

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