Re: focal lengths

[P3D Larry Berlin <lberlin@xxxxxxxxx>]

I Wrote:
> adjust the stereo base (distance between 
>>cameras) to obtain the equivalent separation angle based on 
>>how close the object appears on film and how much parallax is 
>>wanted. Is there a *right* term for this angle?


> John Bercovitz writes:
>I'm a little confused as to these angles you guys are talking 
>about.  Could someone expand on this?  You're not talking about
>using toe-in instead of parallel views, I think.  Something else.

Hi John, 

I did fail to mention the distortions caused by the process.

You said:
>I'm a little confused as to these angles you guys are talking 
>about.  Could someone expand on this?  You're not talking about
>using toe-in instead of parallel views, I think.  Something else.
>
>I mean it sounds like you're talking about vergence but vergence 
>takes place when you're looking at views, not when you're taking
>pictures.  If you had vergence when you took pictures, you'd
>have vergence^2 when you viewed them resulting in the distortion
>of reconstructed space so ably described and calculated by the
>researcher Andrew Woods in his seminal paper on the subject.

The amount of distortion would depend on how much toe-in is needed to frame
the subject. If you are using 1000mm lenses, it would be significant but if
your shot is framed tight enough on the bear so the background and
surroundings are not part of the scene, it would still result in fairly good
images. There is no way to use really long lenses from a distance and have
the cameras stay parallel to each other. It's possible that the flattening
effect of the long lenses would modify the distortions in a way that aids
the sense of depth in the picture so you may not need to have as much
distance between the cameras to get the effect you want. This particular
distortion tends to increase the sense of roundness of the subject in the scene.

If the subject is a wild animal of significant proportion, I'd rather have
the distortions in the image than distortions to my bodily exterior from
getting too close!

With lesser length lenses, you might succeed in keeping the cameras
parallel. Perhaps this would result in the *dollhouse* effect you mentioned.
Maybe there is an optical trick to reducing these distortions. It seems any
kind of imaging through a lens system introduces some level or kind of
distortion. The goal is to obtain an interesting and good image that is
close enough to familiar appearances to be used. If that means working to
achieve a balance of various distortions using tools that operate outside of
*normal* visual ranges, so be it. They could end up being of greater
interest than more normal shots!

Larry Berlin

Email: lberlin@xxxxxxxxx
http://www.sonic.net/~lberlin/


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