[photo-3d] Subject: interocular adjustment in a viewer

[Peter Abrahams <telscope@xxxxxxxxxx>]

   I agree that the Red Button viewer uses mediocre optics, as do the other
viewers I've used, which unfortunately does not include the best modern
models.  I look forward to trying a Saturn viewer, and the de Wijs viewers.
   I don't agree that interocular adjustment is unnecessary.  I admit that
having not used the Saturn viewer makes my objections rather theoretical,
though based on extensive experience with many types of binocular instruments.

>they should be able to comfortably view just about any slide after adjusting
>the viewer lens spacing just once in their lifetime

   I believe the subject of the discussion is viewers that can be used by a
group of people -- clearly one can defend a nonadjustable viewer that is
used by only one person -- and in fact, it could be optimized in many ways.

> small lenses their designers justified by including interocular adjustment

   The small lenses were no doubt a cost - based decision; however the
'justification' is also to allow a longer focal ratio, which directly
permits better control of aberrations.
   The main reason I think adjustable viewers are important is this:
   Viewing through an eyepiece should be done through the center of the
eypiece, the optical axis.  This minimizes aberrations, and if you wear
spectacles when viewing, it maximizes eye relief.
   However, stereo viewers are not usually used this optimum manner, for
two reasons.  1, when using a viewer with non-adjustable lenses, only those
with interpupillary distances that match the interocular distance, will be
able to view directly on axis.  2, a viewer with slides that are mounted at
a different distance than the viewer interocular (or the users IPD) will
also force the line of sight off the optical axis.  (I suppose there are
unusual cases where a skewed line of sight joins the slide, the lens axis,
and the eye's axis)  (The solution that allows IPD adjustment and maintains
alignment is a 2x2 viewer that moves the slides with the eyepieces.)
   The question: is this misalignment to any degree that is objectionable,
or is it a trivial consideration?  Taking the Saturn viewer as an example,
it is advertised as using 40mm lenses, with centers 66 mm apart.  This
means that my IPD of 56mm would place my pupils about 5mm off axis.  These
are very fast f2 lenses, and I'd guess that this is enough to introduce
problems.  They might be hard to see -- but they might be easy to see when
compared to an adjustable Saturn.  This is the nature of quality in visual
instruments -- it is only apparent when a superior example has been
experienced or is available to compare.
   It was also mentioned that large lenses suffer much more from
reflections from lights behind the user; this is a very real consideration
when designing eyepieces, and is reduced by designing the eye lens (outer
lens) to have a convex first surface, and using quality antireflection
coatings.

>I have narrow eye-spacing and setting the red button interocular so that 
>I can even attempt to view all of the image area in 3D means I have
>to painfully diverge my eyes.

   I've also found that a standard Realist mounting with ~62 mm chip
spacing means that those with smaller interpupillary distances have
problems, though mostly or only when using the viewer in a way for which it
was not designed: full frame instead of Realist size chips.
   There is another reason people fiddle with interocular adjustments,
which doesn't change whether it is unnecessary or desirable:  When someone
with narrow eyes views through the inner halves of the viewer lenses (the
half of the lens towards your nose), and does not use the outer halves of
the lenses -- the viewer lenses are acting like the prismatic lenses found
in Holmes viewers, and 'diverging the view'.  (This is also likely to
deteriorate the image quality.)  This is what was described here:

>people adj. the viewer lens spacing whenever they are having trouble fusing
> a stereo image....The lens spacing is trying to correct for changing image
> spacing in the mount, not match your actual eye spacing.

However, the fact that the adjustment is used in this manner does not
reduce its usefulness to those who have nonstandard IPDs.

>Having adj. lens spacing is only an attempt to fix a mounting problem.

I belive this adjustment is far more useful than that & is quite important
for comfortable viewing with a group of people.  However, I will certainly
give a fixed viewer a trial with an open mind.

>I think John Bercovitz made up a nice diagram showing why a viewer does 
>not need adj. lens spacing.

Anyone know where this can be found, so I can disagree with it?

There is another issue here:

>viewer design follows the same standard as the mounting gauge for infinity
spacing.

   Viewer design should follow the photographic art, not dictate the
standards for mounting.  If mounting to infinity is accepted as the best
way to mount, then viewers should be built to those standards.  And we
should all accept the wild diversity of near point and window framing that
this produces.
   My experience is that slides mounted to the window are far more
effective.  I know this is the subject of much debate, and that there are
some reasons to prefer mounting to infinity, and that not all of them are
wrong.  But that's another topic.

   If any Cascade Stereo members own a Saturn or de Wijs viewer & can bring
it to Saturday's meeting, I'd like to try them.
--Peter
_______________________________________
Peter Abrahams   telscope@xxxxxxxxxx   The history of the telescope & 
   the binocular:   http://www.europa.com/~telscope/binotele.htm

 

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