Re: [photo-3d] Base Calculation?

["Dr. George A. Themelis" <drt-3d@xxxxxxx>]

>At a given subject distance, a wider angle lens demands a wider base.
>True or false?

True and false. :)

As others pointed out, "demand" is a strong word.  Now, there
are two ways to see this:  The "narrow-minded" way is to think
in terms of achieving a given (usually maximum) stereoscopic
deviation.  In this case, the Focal Length acts as a magnification
factor.  Wide angle lenses (small FL) make everything look smaller,
including the stereoscopic deviation.  From this perspective,
a wider base is needed to result in the same fixed stereoscopic
deviation.  Long FL lenses magnify things so a smaller base
is needed.

The other way to see this is to ask yourself why are you
using short or long FL lenses?  What are you trying to
photograph?  What is your subject?  Where is your subject?
Think like a photographer, not a mathematician or engineer.

Long focal lenses are used either to concentrate into a
small detail (portrait, detail of building) or to make
things appear closer (animals in the zoo).  Usually both
situations demand that you use LARGER stereo base, not
smaller.  In both cases you have very limited natural
depth range in the scene, either because you are looking
at a small & narrow detail or because your subject is far
away.  Hence McKay's principle that the ratio of FL to
the Stereo Base should be constant, i.e. going from
50mm to 100mm lens you should DOUBLE your stereo base
(separation of the cameras) not cut it in half.

Why do you use a wide angle lens?  Usually because you
want to squeeze as much of the scene as possible into
the film.  Whether you increase or reduce or leave
your stereo base alone would depend IMO on where your
closest to the camera object is.  Usually very wide
lenses are used with things very close to the camera
in which case I would recommend not changing the base.

>Base for 20mm lenses should be ~60mm with a subject 
>at about 1m.  True or false?

Depends on the subject really... Is infinity 
included in the picture?  60mm sound good to me.

George