P3D Tom's problem: Wide interocular?

[fj834@xxxxxxxxxxxxxxxxxxxxx (Dr. George A. Themelis)]

Hi Tom,

>To answer your questions: No I do not wear glasses. My 20/15 vision is
>non-corrected (that means that I see at 20 feet what an "average" person
>sees at 15). 

Ahhhh... Tom!  It is about time that you do something for this problem of
yours.  Surely there is some type of glass prescription that will make you
see at 20 feet what an average person sees at 20 feet!!! :-) :-) :-)

>My interocular spacing is 67.5 mm...
>Since only one other person mentioned having the same problem (in an off
>list message to me) I will have to assume that it is only me that can
>not see the full 5 perf image in a red button viewer.

No.  That makes two of you! :-)

>I was just wondering why? Can you see the full frame (including corners)
>on an 8 perf stereo slide without moving the viewer?

Seriously now, I did a little experimentation and here are my conclusions: 
I can see the 5p (Realist size) without any problem.  I can see 7p but I
might have to do a little bit of "looking around" at the edges.  With 8p,
in order to be able to view the entire scene, I have to "rock" the viewer. 
That is, I slightly tilt (rotate) the viewer around an axis perpendicular
to its plane.  By rotating it CW, I see the right side, by rotating it CCW,
I see the left side.  This "rocking" is very slight and does not cause any
distortions.  And, as a matter of fact, I am convinced that the Realist red
button lenses show more tolerance for off-center viewing, compared to
equivalent (in FL and size) lenses of the Revere and Wollensak lenses whose
image tends to "break down" off center (this phenomenon was explained to us
some time back by an optical engineer in this list).

Why do you have a problem seeing 5p?  The only explanation I come up with,
is that the spacing of your eyes is wider than the infinity separation of
the Realist format.  The Realist format has a near separation of 62.4 mm
and a far separation of 62.4 + 1.2 = 63.6 mm.  Your eye spacing is 4 mm
wider than that.  4 mm is a good fraction of the 20 mm diameter of the
lenses.

Now you are going to say, "if I have a problem with 67.5 mm, how about this
fellow, Peter from the Netherlands, who has 70 mm eye spacing?"  I can only
guess that he has a problem too, possibly to an even greater extent.

In past photo-3d discussions we've heard from people like Paul Talbot who
have the opposite problem:  Too narrow interocular spacing.  Paul has
reported that has a problem seeing the entire 5p view which he can only
achieve by experimenting with the interocular adjustment of the viewer.

Which prompts me to ask Tom:  Where do you set your viewer's interocular
adjustment?  Is that centered or to you tend to go to an extreme setting?

One possible experiment you can do:  Take a 7p slide mounted in a 7p Albion
mount.  Slice the mount in half at the center (vertically), separate the
two halves by 4 mm and tape them together.  Put this (stretched) stereo
mount in the RB and set the interocular at the widest setting.  Does this
make things better?

I am looking forward to Tom's and other people's reports regarding eye
spacing and the effect on viewing stereo slides.

The bottom line for me is this:  Try and have your eyes separated by the
recommended 63-64 mm spacing :-).  If this is impossible and your deviation
is more than a few mm off, then using larger lenses might be the best way
to enjoy stereo slides mounted in Standard Stereo mounts.

Regards -- George Themelis

PS.  The numbers for the infinity separation in the Realist mount should
     not be confused with the spacing of the Realist camera lenses which
     is 70 mm.


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