Re: Waist level finders: 3d?

[T3D Peter Homer <P.J.Homer@xxxxxxxxxxxxxxxxx>]

>  My brother was playing around with my new-old
>Bronica S2A SLR. This is a 70's vintage medium format
>(6x6) SLR with a waist level finder. Mine has
>a 75mm f2.8 lens mounted on it.
>
>  He claimed that the view on the ground glass
>looked 3d. And that if you lined up a near and
>a far object using one eye and then looked with
>the other one you could see that they were no
>longer lined up.
>
>  I may have seen the effect. I found it difficult
>to judge given the limited DOF of the wide open
>lens. It was too dim indoors to stop down the lens
>for more DOF. So I'm not sure if it was real or
>an illusion of some sort.
>
>  If there really is an effect, I assume it is
>from light rays coming through opposite edges
>of the lens opening and hitting the ground glass
>at the same spot. But the ground glass isn't a
>perfect diffuser so you will see the ray that
>is aimed towards your eye better than the one
>from the opposite side of the lens which is
>aimed slightly away.
>
>  If this is true than stopping down the lens
>would reduce the effect since the with of
>the lens opening is reduced.
>
>So is this possible or is my brother
>hallucinating?
>
>Thanks - Greg E.

The large lens of this camera coupled with the large waist level finder
would seem to make it possible to use both eyes to look through the lens
and so get a stereo view. But the ground glass screen might be thought to
prevent this ,however in the last century there was a dvice invented by
Claudet I think called the stereomonoscope. This was a stereoscope with two
lenses as normal at first but producing a real image on a ground glass
screen which was then viewed by a single large lens. This produced a stereo
view apparently without the need for a viewer in the conventional sense but
still with a very limited viewing window only one person could see it at a
time. This tends to suggest that you can get a stereo image through a
ground glass if its only slightly ground so the rays dont lose direction.
In a fairly recent edition of Stereo World there was an article which
featured someone who had re-invented this device who described how he had
to lightly smear the ground glass with castor oil in order to get it to
work. Because the refractive index of the oil would be closer to the glass
than that of air this would have reduced the frosting so that the rays were
scatterd less and their direction not lost.
  In fact its hard to see what the ground glass is for unless it means that
the first real images once focussed on it should be viewable by anyone with
only the need to focuss the second virtual image by movements of the head.
Or perhaps the inventors did not realise that an aerial real image was all
that was neccessary.
 Because again in the last century James Clerk Maxwell produced his "real
image stereoscope" which did away with the screen . This was also
"reinvented" by Jeremy Hinton (who is a member of P-3D and perhaps this
group) after reading an article on optics . Also Colin Clay has made his
own version of Maxwells device which I have seen and it reqires no focusing
once set up.  P.J.Homer



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