[photo-3d] R: . Re: Camera separation in stereo photography

["Sergio Baldissara" <winter@xxxxxxxxxx>]

Ciao!
While reading this thread (when does the 1/30th rule apply?) I realized most
people speak about focal lenght but seem to forget the film frame format.
It should be useless to remind the 1/30th formula approximates the more
accurate formula:
Base = (distance from Near point) x (max allowed Deviation of homologous
points [on film]) / (Focal lenght), say b=n*d/f.
"d allowed deviation" is usually said to be1,2 mm (36mm/30) on 135 film
(24x36mm frame), and 3mm (9cm/30) on 120 film (6x9cm frame), in other words
roughly 2 degree parallax.
In our experience we realize those figures were set very cautiously, as
1,2mm usually also works for Realist format, and 3mm for 6x6 format. In both
cases horizontal deviation substends a parallax of about  3 degrees.
Can we dare more? It's a difficult question to answer.
Fusion capacity physiologically varies between individuals: some skilled
stereo watchers are even able to freeview stereocards in parallel format.
But... (as usual I'm heretic) we mustn't confy on the observers' ability to
fuse (some 3d photographers sometimes do so, and even seem to challenge the
skill of their audience).
Indeed the only thing that really matters is the ability of our stereos to
capture other people's  eyes and drive their brains to stereo vision.
When we display pics to other stereoscopists, we deal with people willing to
fuse, skilled in fusion, even not caring about some little eyestrain.
But if we want to show our work to laymen, we must be more cautious: we need
to capture the automatic fusion of 100% of our audience... unless we want to
keep stereoscopy as an exoteric art...
ciao dall'Italia!
Sergio