P3D Quantifying Stretch

[Tom Hubin <thubin@xxxxxxxxx>]

Greg Erker wrote:
> 
> >>The 43mm FL is also nearly ortho for Red Button
> >and similar viewers (John B would love it :)
> >
> >Does that mean that most vintage stereo camera lenses (with focal
> >lengths of 35 - 37mm) don't produce 'ortho' images when viewed in the
> >Realist (or similar) viewer?
> 
>   Yup. The RB produces about 26% stretch (44/35) on
> shots taken with a 35mm lens. Some people actually
> have preferred this "accentuated" depth in my Realist
> slides in RB vs nearly ortho medium format shots
> (82mm viewing lens, 80 or 75mm camera).

Hello Greg,

I have a question on your usage of a numerical value for stretch. 

The lateral magnification is normally what we simply call magnification.
It is the side to side or top to bottom magnification in a plane. The
lateral magnification is proportionate to the lens focal length for
images at infinity like those seen in a simple viewer.

The longitudinal magnification is the magnification along the optical
axis. The longitudinal magnification for normal images is the square of
the lateral magnification.

When the lateral magnification is one so also is the longitudinal
magnification one. Hence no observed longitudinal distortion known as
stretch. When the lateral magnification is 44/35 or 1.257 then the
longitudinal magnification is 1.580. So I would be inclined to say the
stretch is 1.58 or 58% rather than 1.26 or 26%. I beleive this squaring
is what makes stretch so noticable.

Is there a standard or accepted definition for quantifying stretch?

Tom Hubin
thubin@xxxxxxxxx



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