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

[abram.klooswyk@xxxxxx]

Koo Ferwerda's remarks on the extra-axial usability of  
viewer lenses has been quoted in this thread. 

L.E.W. van Albada (1868-1955) has given this some thought in 
the 1920-30s. He was the designer of the famous viewfinder 
known by his name. He was advisor of the Dutch optical firm 
Oude Delft (later known as Oldelft, and in recent years in 
the news for having sold military optical equipment to regimes 
like that in Iraq).

But van Albada was most interested in stereoscopy.  
He calculated and built himself several highly corrected two 
and three component wide-angle oculars for stereoscopes. 
He also grounded the lenses himself. Koo Ferwerda has 
written about him in a 1961 article in a Dutch photography 
magazine.

In 1931 van Albada, probably on invitation by von Rohr, wrote 
the stereoscopy part of the multi volume German "Handbuch der 
Wissenschaftlichen und Angewandten Photographie" (Textbook 
of Scientific and Applied Photography).

There he said: "The best possible reproduction of stereo 
images is achieved (...) [if] both pictures are separated and 
laid each on a sliding carrier which is joint with the lens, 
so that their [the lenses's] central points can be centered 
exactly, and brought into the right distance fro each other 
(eye distance of the observer)."
For the present day this means adopting the 2 x 2 x 2 system 
for 35 mm slides, and in fact Koo Ferwerda has used that 
system himself, but mostly with the superslide format, 
trimmed from Rolleidoscope pictures.

But van Albada (1931) continued: "... [if] the distance of 
two corresponding distant image points (...) is either 
greater or smaller than the eye distance and not changeable 
(...) [then] one should bring the centers of the stereoscope 
lenses in the same distance from each other as the mentioned 
image points have, because only in this way far distant 
objects are looked at with parallel eye axes. (...) 
In general good stereoscope lenses admit an eccentric use 
(...) of a few millimeters [einigen wenigen mm] from the 
lens centers without perceptible disadvantages."

When van Albada says so it must be true, since he was so 
aware of optical quality.
But the "wenigen mm" are of paramount significance for users 
of joined stereopairs, like the Realist family, and the 
RBT family, see the following.

In the same paper for the 1975 ISU Congress I had reported on 
literature figures of interpupillary distances. Of the world 
population of adults probably over 95 % have between 55 and 75 
mm interpupillary, and nearly 99 % between 50 and 80 mm.

When the far point separation on slides is about 65 mm,  
stereoscope lenses which permit an eccentrical use of 5 mm cover 
interpupillaries from 55 to 75 mm (65 minus and plus 10). 
Only the small number of people falling outside this range 
might notice some image degrading. 

Therefor stereoscope oculars should be designed specifically 
for eccentrical use, so be large, and they should not be 
compared with other eyepieces like those for binoculars and 
microscopes, where eccentric use has no purpose.

The Frenchman Dubosc was probably the first to try this; in a 
Dutch translation (1862) of a French textbook the description 
of Dubosc's design says: "(...) the lenses, which are moreover 
broad (...). This alteration, by Dubosc, makes that how far 
the eyes may stand apart, the seeing is still good." 

Abram Klooswyk


 

Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/