Re: Viewer interocular & vanishing mounting errors

[P3D <PTWW@xxxxxxx>]

Regarding my 100% field of view in each eye method of setting the
interocular spacing on the RB viewer, DrT wrote:

>Apparently you are adjusting the viewer to maximize your field of view.

To maximize my stereoscopic field of view, I guess you could say.

>Does you method change when you take your glasses off?

I rarely use the viewer without glasses, because a) the focus range is
not quite large enough; b) my glasses also correct for astigmatism,
which the viewer probably cannot do; c) as is probably true for most
eyeglass wearers, my left and right eyes require differing degrees of
correction, and the RB lenses adjust in synch with each other.  It
seems, however, that the increased image size due to the decreased
effective FL of the lenses roughly offsets the effect of moving my
eyes closer to the viewer.  I'm left with still having to set the
lever to its extreme narrow setting to see 100% of each eye's view.

> Do you adjust it for each individual slide?

Only when a problem arises.  The most common example, as discussed below,
can be traced to mounting errors.  When the mounting problem is corrected
the slide can be viewed with my normal interocular setting.  However, I
do have at least one slide that I cannot view properly with my normal
interocular setting, and I cannot determine exactly why that is so.  I
currently suspect there is too much depth (under 7 ft to infinity), and
mounting to the window makes the infinity separation too large.

>What is your eye-spacing?

I asked my optician what they had in their records, and they gave me two
numbers, an "inside" and "outside."  I don't know what that means.  As
best as I can tell, my center-to-center IPD is about 55 or 56 mm.  I
have to buy the smallest sized adult eyeglass frames.

>Apparently you are viewing slides with some convergence even at infinity.
>Most people use it like that.  I used to do that and then realized that
>bringing the lever closer to the center was OK too and some times better...

Sometimes bringing the lever closer to center is better for me, too,
because it lets me pretend I don't have a mounting problem with the slide!
But it comes at a cost: if you cannot see 100% of each image with the
appropriate eye, you cannot see stereoscopic depth throughout 100% of
the scene.  As an extreme example, consider what would happen if a
doughnut-shaped opaque object were placed in front of the left side of
the stereo pair, such that only the inner 50% of the left image is seen
by the left eye.  You will see stereoscopic depth in only the inner 50%
of the overall scene that is viewed with both eyes.  The outer 50% would
be 2D.  This is effectively what happens if you set the interocular wider
than the setting that allows you to see 100% of each eye's view, though
you might not readily realize it because your brain will be doing its best
to fill in the gaps utilizing other depth cues

>You mentioned earlier that this method gets rid of misalignment.  What
>misalignment is that?

I think I'm being misquoted.  What I said was:
>>I have noticed that some mounting problems can seem to magically
>>disappear as the interocular is changed.

Perhaps the confusion is because the tilt/alignment discussion came up
about the same time.  Here is the "most common" situation that I referred
to above: take a slide with a window violation in the lower right edge.
It has to be a side-window violation: where an object is coming through
the window, but intersecting with the window's vertical edge.  If the
interocular is set so that I see 100% of each eye's view, the right
hand vertical edge of the window will not fuse.  Instead, I see a window
that looks something like this:

          ........
          |      |
          |      |
          |      |
          |     / \
          ......|.|


(Disgracefully crude by Gabriel's standards, I know!  I hope this at
least gets the idea across.)

By moving the interocular lever outward, the right hand window can be
made to *seemingly* fuse normally because the split in the vertical
frame goes away.  But it is *not* fusing normally!  Because only one eye
is seeing the right vertical frame, there is nothing to fuse; the window
violation seems to vanish.  But this slide will not project well!

I have come to realize that the biggest reason I previously thought
Kodalux mounting was just fine is that the interocular of the $3 viewer
is much too wide for me.  Viewing the same slides in the (dimmer ;))
T'd RB, *with the interocular set as I described,* allowed me to finally
detect many such window violations in my Kodalux-mounted slides.

Paul Talbot


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