P3D 3D Movies and the 1 to 30 rule

[Jonathan Gross <jonathan.gross@xxxxxxxxxxxx>]

Larry Berlin wrote:

> >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.
> 
> *****   I'm sorry to disagree here but this list of numbers seems oddly
> reversed. 1:42 is 30% LESS interocular separation ...

Yes, Larry you are correct; it is 28.6% less separation.  I noticed my
error after I sent the posting, and didn't think it was worth an
unsolicited correction.  Now that you have brought the issue up,
however, the ratios are correct as stated.

Anybody who has tried to steady a telephoto lens knows that the longer
the focal length, the greater the result on the image from the same
movement of the camera.  For a constant image format, the longer the
focal length, the smaller the lens base for a given widow distance. 
Inversely, with a constant image format, a longer focal length will
place the stereo window farther away for a given lens base.


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