Re: 6x6 and lenses shorter than 80 mms

[P3D Greg Erker <erker@xxxxxxxxxxxxxx>]

>I would simply freeview the transparencies on a light table
>and the prints under a good light either parallel or cross-
>eyed.
>
>        Does this solve the orthoscopic problem?  If so what
>is the relationship of our eyes to focal length?  To me, I would
>understand our eyes to have a variable focal length.  Is this true?

  When you view it you want the angles subtended by
objects to be the same as in the original scene.
With a transparency this is achieved by viewing from
the same distance away as the lens was from the film plane
in the camera (John B could tell you which part of the
lens, but let's just pretend it is a pinhole 65 mm from
the film plane).

  A normal human can't focus that close (65 mm) so you
place a lens between your eye and the transparancy to
bring it into focus. If you use a 65mm lens and place
it correctly your eyes focus at infinity while seeing
the transparancy sharply in focus. And the original
angles of the scene are reproduced correctly.

  If you freeview from 10" away (250 mm) then you
are roughly 4 times as far away as you should and thus
all the angles are 1/4 what they should be. The depth
in the scene will be stretched by a factor of 4 also.

  With a print the correct viewing distance is the
original FL times the print magnification. Thus a 4x4"
print taken with your 65 mm lens should be viewed from
130 mm away. An 8x8" print from 260 mm away (close to
the 10" mentioned above).

  So I would say that unless you are freeviewing large
prints you are quite far away from ortho viewing conditions.
You should invest in some lenses (achromats preferably)
even if 80 or 90 mm to get closer to ortho with your
slides. They will be wonderful to behold.

BTW check out Robert (RJ) Thorpe's web site that shows
several home built MF viewers, including mine:
http://www.skep.com/3d/gallery.htm

  To answer your final question, yes I have heard that
the eye's focal length changes as it focuses. But I don't
think it is directly relevant to the issue at hand.
To repeat, ortho is achieved when angles in the scene are
reproduced the correct size. The focal length of your eye
isn't directly involved.

Greg E.
Saskatoon, Saskatchewan, Canada



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