P3D Re: 3D Movies and the 1 to 30 rule

[Jonathan Gross <jonathan.gross@xxxxxxxxxxxx>]

> 
> ... use the 1 in 30 rule and readjust the stereo base for each shot.
>
I would like to point out that the 1 to 30 rule is only valid under the
following conditions:

1.  The focal length of the lenses are about 35mm
2.  The film (or imaging element) is 24mm high
3.  The image separation at infinity (delta) is 1.2mm 

If any of these conditions do not hold, then the 1:30 rule does not
work, although there are other combinations of image size, delta, and
focal lenght which also yield a 1:30 ratio. When using 35mm SLR cameras,
for example, here are the calculated ratios for a delta of 1.2mm:

Lens Focal Length     Ratio of Lens Base to Stereo Window
	24mm			1:20
	35mm			1:30
	50mm			1:42
	100mm			1:83

This explains why the standard 1:30 rule works for Realist cameras, but
not for SLR's with standard 50mm lenses; for 50mm lenses the ratio is 1
to 42, and not 1 to 30, a 40% greater seperation.

If your imaging element size, focal length, and delta are different,
then the calculations are different.  I've used the formulas in the book
"Stereo Photography" by Fritz G. Waack DGph AFIAP, and they have always
worked for both Hyper- and Hypo- (macro) stereo.

Why would the ratios for Hyper- and Hypo- stereo be different?  Because
for normal and hyperstereo the distance from the lens to the stereo
window is so much larger than the distance from the lens to the image
(film) that for all practical purposes, it can be ignored.  With macro
stereo, however, when the distance to the stereo window starts to
approach the focal length of the lens, then this factor starts to take
on significance.  The result is that the center of each image is no
longer directly behind the center of each lens, but is instead "shifted"
to the outside of the pair.  As a result, the lens base must be slightly
smaller to compensate, and keep the images "centered".  This is why the
lenses in a Macro Realist are closer than the 1:30 rule, but the camera
still works fine.

Jon Gross


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