P3D Re: What is the "Stereo-Base"?

[Bruce Springsteen <bsspringsteen@xxxxxxxxx>]

How delightful!

David B of Tel Aviv wrote:

> What is the "stereo-Base"?

The cause of a great deal of debate, sir!

> Some people would say: 65mm, like the distance between the
> eyes. And some would say: 1/30 from the mearest object.
> *Rubbish* - Those answers is good for amateurs!
> I found this formula provided by David Kuntz in the ISU STEREOSCOPY:
> 
> i   *   b  
> ----------     =    n
>     p

As it happens, the subject of stereo base has been heavily thrashed on
this mail list this year, David.  You have innocently wandered into a
discussion that makes the political differences in your part of the
world look mild! ;-)  Congratulations!

Let me suggest that there is general agreement on the following points:

1.  You do *not* need stereo base equations to make good stereo
pictures.  At least not in the vast, vast majority of situations.

2.  65mm is perfectly correct in most situations, that being the
approximate average distance between the pupils of human beings (who
are the sole practitioners of stereo photography on this planet ;-) ).
 When combined with viewing lenses that match the focal length of the
camera lenses (or nearly so) you achieve what is called "ortho-stereo"
- which is viewing the scene just as you would with your own eyes in
the place where the picture was taken.  You can't do better than that!

3.  For the minority of situations where a stereo base larger or
smaller than the 65mm average is used, there are two common approaches:

a)  Trial and Error.  You simply move the camera a distance that you
think might yield an acceptable result - less for close subjects, more
for distant ones.  Several pictures taken at several base widths gives
you more combinations to choose from later - improving the chances of
a pair you like.  Many excellent stereo photographers use this method
almost exclusively with excellent results.

b)  1 in 30.  This is the most-often mentioned rule for calculating a
result quickly and easily.  It is based in good solid theory,
simplified for ready use in most situation.  It is designed to provide
a good stereo effect under most conditions, without ending up with
*too much* distance between shots (a serious problem, not to be taken
lightly.) 

4.  Very many formulas have been suggested to achieve a variety of
other goals in shooting stereo.  In general, these are of more
theoretical than practical interest, though any one may be ideal for a
particular situation or goal.  The study of these numerous methods,
including the one you mention, is potentially very time-consuming. 
Many would suggest that your time would be better spent on learning
other aspects of the art and science of stereography.  Others enjoy
the challenge, the understanding of the theoretical underpinnings, or
experimenting with and comparing results using these different
calculations.  This is a matter for personal choice, but not purely a
matter of opinion - each approach has definite merits and drawbacks. 
But even the people who are most interested in the math generally
admit that they do not need to use it to make good pictures - and
won't suggest that anyone else must.

5.  A most comprehensive attempt to describe the general geometry of
stereo photography, with all variables accounted for in the formula,
was undertaken by John Bercovitz and Steve Spicer in another article
in "Stereoscopy" in late 1996.  There is a web site which contains the
text of that article - I do not have the URL on hand.  If you like
math and/or theory for their own sake, you may wish to read that
material.

6.  "Don't worry - shoot happy!" 

Others will give direct answers to your questions about parallax and
metric measure (A and B).  Your question C has many good answers, it
will take time for you to learn them all.  Best of luck!

Bruce Springsteen
Milwaukee, Wisconsin, USA 
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