P3D Stereo base formula: Please help

[Dogbreath <hopi@xxxxxxxxxx>]

Hi.

Could someone please refresh my memory on the formula stereo photographers
use to calculate the length of the stereo base when using a single-lensed
camera (ie, a 35mm SLR)? For example, if I'd like to shoot a macro stereo
pair using a 50mm lens and my subject is ten inches away, how would I
calculate the appropriate distance to slide the camera from the one side of
the base to the other? I've read the simple 1/30th rule, but that doesn't
seem to take the focal length of the lens into account (or, if it does,
it's based on the 35mm lens of the Realist and other 5 perf cameras). Are
there any limits? Is there a point where the formula fails? I've read about
the mysterious macro stereo Realist, and as far as I can tell, it's base
must have been about an inch to an inch-and-a-half (a pair of 3.5 lenses
couldn't be set together much closer than that). That would seem to be an
innapropriately long base for macro work approaching 1:2 magnification, but
if the camera was designed for and used by doctors (dentists, mainly), it
must have worked as intended.

Thanks for your time.

DB



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