P3D Re: "Bowl" distortion in viewers

[erker@xxxxxxxxxxxxxx (Greg Erker)]

>The funny thing is that this "bowl effect" (a.k.a. "Larry Berlin Illusion")
>is not supported by theory.  In order to see a 3-dimensional distortion you
>must have two slightly different images.  Assuming that the lenses are
>nearly identical, why would the two images be different?  Any distortion
>in one lens matches the distortion in the other, making this a 2-d effect.
>No matter how bad the lenses are, if they are identical then there is no
>3-d distortion.  Small difference between lenses do exist (like a
>difference in FL) buy they do not necessarily lead to fish-bowl distortion.

  Suppose your eye spacing matches the infinity
separation of the slide and you adjust the interocular
spacing on the viewer so that you are looking through
the center of each lens. Both eyes see the same.

  But for a near object you aren't looking through
the same points of each lens (since the near image
separation is less). Thus if the lenses have barrel
or pincushion distortion the relative placement of
the near object will be shifted, shifting its apparent
3d depth.

  So I think it can happen. But I haven't seen it
myself, either because I'm insensitive to it or
because my MF viewer has too little distortion.
(Possibly true since a 50x50 mm slide in MF is
about like viewing a 5 perf realist slide. Larry
was talking about viewing a 7 perf or full frame
image thru the RB.)

Greg E.



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