The FORMULA

[Paul S. Boyer <boyer@xxxxxxxxxxxxx>]

Rafel Botey Agusti <rafel@xxxxxxxxxxxx> asks "what is the formula?"
It is Kitrosser's formula (1953, in Photographic Society of America
Journal, Section B: Photographic Science & Technique 19B, no. 2, pp.
74-76).
0.0635*w/F*W*I=1/d-1/D
where w is the width of the negative or diapositive, F is the
lens-to-film distance, I is the interlens separation, d is the
distance to the nearest object, and D is the distance to the farthest
object in the field of view.  (All distances in meters.)  W is a
constant, set by Kitrosser at 1.52; I think that in actual practice
it should be  more like 1.25.
If you wish to have objects at infinity in your picture, then 1/D =
0.  If the window is at 2.1 m (as in the Realist), set 1/d at 0.476.
Then
w/F*I = 9.374
For full frame, w=0.036 m;
for European format, w=0.031 m;
for Realist format, w=0.023 m.
Assuming that you are using full frame,
F*I = 0.00384
If your lens is focal length 50 mm = 0.05 m, then the lens-to-film
distance will be roughly 0.05 m, so
I=0.0768 m = 76.8 mm
Good luck measuring the tenth of a mm, but of course such precision
is unnecessary.
Note that if you use a shorter focal-length lenses, you can tolerate
a wider separation.  For example, if in this case you used two 35-mm
lenses,
F*I = 0.00384
I = 0.00384/0.035 = 0.11 m = 110 mm.
That might give you more room to have two cameras side-by-side
without exagerating the stereo effect too much.
[Note to readers: Hope I didn't make any math errors this time!]
Paul S. Boyer
New Jersey, USA
<boyer@xxxxxxxxxxxxx>


------------------------------