[photo-3d] Brewster and Wheatstone on the stereo base 3/5

[Abram Klooswyk <abram.klooswyk@xxxxxx>]

In France the Abbé Moigno, editor of the journal Cosmos, in a 
discussion of binocular cameras (Quinet had made one which 
resembled the one described by Brewster) wrote that they would 
give "assez beau résultats" (rather beautiful results) in the 
reproduction of small and medium objects. But "evidemment" 
(obviously) they would not give the desired stereoscopic 
effect for very large objects or landscapes. To this Brewster 
answered (p. 147): "This criticism on the limitation of the 
camera is wholly incorrect; and it will be made apparent, in a 
future part of the Chapter, that for objects of all sizes and 
at all distances the binocular camera gives the very 
representations which we see, and that other methods, 
referred to as superior, give unreal and untruthful pictures, 
for the purpose of producing a startling relief."

Then Brewster criticizes the statement by the Abbé that the 
angles at which the pictures should be taken "are too vaguely 
indicated by theory", and also Claudet's assertion "that there 
cannot be any rule for fixing the binocular angle of camera 
obscuras. It is a matter of taste and artistic illusion". 

To the latter Brewster says: "No question of science can be a 
matter of taste, and no illusion can be artistic which is a 
misinterpretation of nature." (p. 147)

This sounds almost as a religious statement (BTW, in 
Brewster's opinion his theological works were far more 
important than any of his scientific investigations or 
inventions). It reminds of Boris Starosta's words (May 24, 
2000): >(...) for me shooting with something close to a 
"normal" >interaxial spacing (i.e. close to 2.5 inches) with a 
>"normal lens" (close to 50mm f.l. for 135 film) was 
>something of a religion! 
Claudet's words however seem to be reflected in Ray Moxom's
and Lincoln Kamm's opinions.

Brewster continues (p.148): "When the artist has not a 
binocular camera he must place his single camera successively 
in such positions that the axis of his lens may have the 
directions EL, EL' making an angle equal to LCL', the angle 
which the distance between the eyes subtends at the distance 
of the sitter from the lenses. This angle is found by the 
following formula:-
                1/2 d       1.25
 Tang. 1/2 A = -------  =  ------  
                  D           D
d being the distance between the eyes, D the distance of the
sitter, and A the angle which the distance between the eyes,
= 2.5, subtends at the distance of the sitter."

Then follows a table in which angles "formed by the two
directions of the Camera" for a number of distances "of 
Camera from sitter" are given. It starts with:
               5 inches  .  .  .  28 degrees 6'
and gives all angles, up to 20 inches for every inch, then 
from 24 inches (2 feet) to 60 inches (5 feet) every 6 inches, 
and thereafter every 12 inches (every feet) up to 10 feet.
The "greater part" of the table "can be of use only when we 
wish to take binocular pictures of small objects (...).
In photographic portraiture they are of no use."
"The correct angle for a distance of six feet must not exceed 
two degrees, - for a distance of eight feet, one and a half 
degrees, and for a distance of ten feet, one and a fifth
degree."

In my opinion this all means that Brewster wants the resulting 
convergence of the two successive positions of the camera 
optical axis _always_ to be the same as the convergence which 
the eyes would have in looking at "the sitter" from the same 
distance. ("Sitter" seems to include all subjects.)

This is of course not comparable to the one-in-thirty rule,
which will give the _same_ angle of about two degrees at
_all_ distances. (Distance to the nearpoint, but Brewster
mentions here only instances of one object or one "sitter".)

Abram Klooswyk