P3D Wide angle Stereo Photography

["Dr. George A. Themelis" <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

John Goodman wonders about the relationship between hyperstereo and 
stretch and asks:

>How imperfectly would narrowing the stereo base from the 
>normal ~65mm compensate for the stretch due to using 
>~50mm focal length lenses to view a stereo shot with 
>28mm lenses? 

That's an interesting question.  Actually, it is the exact opposite 
that has been used widely with excellent results:  *Increase* the 
focal length and the stereo base.  You are then forced to stay away 
from your subject.  This results in a picture which appears to have 
been shot from a much closer distance with a normal FL and stereo 
base.  Examples of good applications: Wildlife & action 3D.  But there 
is a catch:  Objects further away appear to be larger in size than 
close by objects.  But the longer FL helps isolate the subject and 
restrict the depth range so this deformation is not very obvious.

Let's see now what happens if we follow the opposite strategy, reduce 
the focal length and the stereo base (maintaining the same ratio).  We 
are then forced to come close to the subject to get some depth and 
detail.  This results in a picture which appears to have been shot 
further away with a normal FL and stereo base.  So the reduced stereo 
base compensates for the wide angle, except what was before a minor 
deformation that no one could see, is now a very serious problem:  
Nearby objects appear to be much larger than further away objects and 
dominate the picture.  If that was a portrait, the "victim" would 
appear to have a really huge nose and very small ears.

One might ask "why are we forced to come close to the subject?"  If we 
don't, then we will get very little depth.  By reducing the focal 
length *and* the stereo base, we minimize the stereoscopic deviations 
(which is what contributes to the sense of depth), per the formula:

Deviations = Focal length x Stereo Base / Distance

So the only hope is to reduce the distance to get some decent depth.  
Following the opposite strategy, increasing the FL and SB and then 
increasing the distance, seems to work much better at the end.

It appears to me that wide angle lenses are best used with normal or 
longer stereo bases, with the resulting "stretch" being the desired 
effect.  In a normal scenic there is usually a lot of sky and ground 
included, but if the scene has a limited depth range (or with the 
proper composition) a wider base will help when a wide angle lens is 
used.  

In any case, it is worth experimenting with different focal lengths 
and stereo bases.  And also, it is worth studying Ferwerda's "World of 
3D" which treats these subjects in great detail.

George Themelis