Re: [photo-3d] Base Calculator

["Dr. George A. Themelis" <drt-3d@xxxxxxx>]

OK, a bit more serious now...  One way to understand
what this 1.2mm means is the following:

In two identical pairs there is no depth.  No difference
between right and left.  p=0

In a real stereo pair there are small differences.  If you 
line up the two images (say in projection) you will notice
that if you align the foreground then the background is
off (shifted horizontally).  The maximum shift is at
infinity. This is p.

Assume that p, measured on the film, is 1.2mm.  Let's
say you are projecting this pair to a vertical size of
50 inches (quite typical for Realist and 50 inch screens
or European format with 70 inch screens).  Since
the height of the image is about 1 inch on the film,
this means you have magnified your image 50 times.
The (on film deviation) p is also magnified 50 times.
50 x 1.2mm = 60mm

Now, 60mm is approximately the spacing of the eyes.
So it will be no problem for the eyes to view the
60mm on-screen deviation.

If your p was 2mm instead of 1.2mm then it would
be magnified to 100mm.  Since this is wider than the
spacing of the eyes, your audience will have to diverge
their eyes to look at the infinity parts of your picture.
Most people would not be able to diverge their eyes
to fuse your distant background.

Now your realize that this is a convention.  If small
screens are used, this would not be a problem.  It
would also not be a problem if the pair is viewed in
a viewer.

But a limit has to be placed somewhere...  This limit
is arbitrary placed at 1.2mm.  This applies for 35mm
film only.  Larger format have a proportionally higher
limit.  

The focal length has nothing to do with this conventional
limit. But it does affect the magnitude of p, all other things
being equal.

George