Re: PHOTO-3D digest 1377

[P3D Neil Harrington <nharrington@xxxxxxxxxxxxx>]

John Bercovitz writes:

>I said:
>
>> The axes of the lenses themselves are parallel or you would 
>> indeed get keystoning.
>
>Now why'd I have to go and say _that_?  If you swing a lens axis, 
>you swing the plane of best focus relative to the plane of the
>film: the two planes are no longer parallel.  If you want to
>get keystoning, you have to swing the film plane relative to
>the direction the camera is looking in.  (And then you would
>consequently very likely have to swing the lens to get a sharp 
>image across the film plane.)  

Dang, John, you're right.  Gentlemanly of you not to mention that the
original mistake was mine, not yours.   :-)

I guess I must have been thinking of the rather severe keystoning that
results from using a Pentax (or similar) mirror device to put a stereo pair
on one frame.  But there the geometry is entirely different, nothing at all
to do with a two-lens stereo camera no matter how the lenses were swung.  I
was all wet.

>So: To avoid keystone error in a 3D pair, keep the image planes 
>parallel.  To make the planes of focus coincident, make sure the 
>lens axes are parallel.

Just so.


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