P3D Re: Stereo base formula

[aifxtony@xxxxxxx (Tony Alderson)]

>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)?<

Hoo Boy! Have you ever seen the Buster Keaton film "Cops"?  Buster
inadvertantly and innocently tosses an anarchist's bomb into the middle of
a police parade. Chaos ensues.

>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).<

This is exactly correct. The 1/30th rule assumes a 35mm lens and a far
plane at infinity. And it assumes the near point will be at the stereo
window. If you're using a different focal length, and/or your far point is
closer than 200 yards, and/or you've got things coming through (not
"breaking"!) the window, this rule of thumb may not be appropriate. Get
thee over to Tech 3D, or just point your browser to the Kiewa Valley web
site for a full discussion and a nifty Excel spreadsheet:

http://www.werple.net.au/~kiewavly/bases.html

>Is there a point where the formula fails?<

Yep. It only solves the maximum allowable base for the usual conditions of
projection, given a certain focal length and depth range. It says nothing
about art. In other words, the maximum allowable base might not give a
pleasing shape to your subject. The formula gives you a range to work in,
but you still have to make a judgement. You might consider bracketing your
base and choose the best pair after processing.

>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.<

I haven't got my reference material handy, but if memory serves, the base
of the Macro Realist is more like 3/4 inch. Yes, even that can produce
noticable stretch. This was not a bad thing for its design purpose of
dental stereography: exaggeration helped make things clearer.
Photogrammetry commonly uses large baselines to make measurements easier.
But the stretch is often not objectionable, as we are not very familiar
with the shape of tiny things anyway.

Hope this helps.

Tony Alderson



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