P3D Re: Divergent Infinity

[abram klooswyk <abram.klooswyk@xxxxxx>]

Tony Alderson wrote (21 Sep 1998 p3d digest 2987):

>Many of the slides, especially the 2x2x2 pairs, had excessive
>parallax. At least one slide (very nice photographically...) 
>had far points over a foot apart on the screen.  Now, by the 
>common standards, this should have been "unviewable."  
>It was not, however.
(...)

What Tony says seems to bear on projection rules (again? sorry). When
you have Koo Ferwerda's book (Jac. G. Ferwerda "The World of 3-D", 
2d ed 3-d Book productions, Borger 1987,
http://stereoscopy.com/3d-books/)
you find an at length discussion at p.222 through 228. In summary, Koo
mentioned: 1. window-on-the-screen-rule, 2. normal infinity rule,
3.infinity-on-screen rule (superimpose far points on screen), 
4.superimpose parts causing ghosting, 5.make NO adjustments.
Koo favored rule 2, Hugo de Wijs does this too. 

Many projectionist however use rule 1, like Pat Whitehouse did.
This is perhaps the easiest to do: just superimpose the images of the
left and the right mount aperture on the screen. The cyclopean stereo 
window is percepted as lying on the screen.

A little calculation makes clear what this means with large screens. 
Suppose a properly mounted 5P slide, far points (e.g. mountains) at
infinity, some near point at the stereowindow, window deviation 1.2 mm 
(which is the standard, the difference between the separation of 
infinity points and of the mount apertures).
When the slide is projected with the window-on-the-screen and the on-
screen-infinity separation at the same time is about 65 mm (2.5 inches),
then the projection magnification is 65/1.2= 54 times.
The width of the screen image then is 54 * the 5P aperture width, 
or 1,135 mm, 44 inch or 3.5 feet. This is small for a large room.
 
It's obvious that in doubling the width of the projected image, keeping
the window on the screen, on screen infinity separation necessarily also
is doubled to 130 mm, 5 inches, which results in divergence when you 
look at infinity, while the amount of divergence depends on how far you 
sit from the screen (and at little on your interpupillary distance).

Now is this a problem? No. 
First: what the hell do you make *stereo* slides for if people should 
look at *infinity*, where there is NO stereo effect?
Second: it does not necessary mean that the divergence all is
*physical*,
i.e. wall-eyed position of the eyes. The brain also manages to "shift
images" horizontally to "superimpose" them, to simplify a complex and
not fully understood fusion mechanism.

Pat Whitehouse said it more civilised (and with a little ironic humor):
the most interesting subject of my slides is mostly not at infinity
(sure it wasn't!), and I change slides so fast (she did!) that people 
don't get the time to look elsewhere.

Now for the perception, what Tony *actually* wanted to discuss :-).
It should be understood that distance perception is different from
depth perception.
Distance: how far is it from me. Depth: which one of two objects is
nearer.

On 22 Aug 1998 in P3d Digest 2917 I stated that the "PePax principle is
incomplete" and I mentioned the Convergence Threshold which I have 
postulated (for what it's worth).

I stated (from general experience, no real tests done):
Differences between photographic and viewing convergence of less
than two degrees are generally not perceptible. Differences of more
than 2 or 3 degrees can become significant for perception, and larger
differences mostly have a remarkable effect.

Sitting at 6 meter (20 feet) from the screen (I suppose a large room)
looking with your 60 mm interpupillary at a separation of one foot 
(300 mm) your divergence is less then 2.5 degrees.
So I mean 
a. This is psychophysiologically no problem. 
b. The distance of the far point will probably not be perceptibly 
   different from looking at a slide with 65 mm on-screen separation.

So far for the distance perception, but Tony Alderson also wrote:

>Anyway, the really interesting thing is that slides with an infinity
>separation of a foot didn't seem any deeper than slides with two and a
>half >inches.  So is infinity infinity, and further separation buys 
>nothing but increasing eyestrain? It would be interesting to compare 
>an "excessive" slide side-by-side with a "standard" slide, to see if 
>this is really true. (it's hard to tell in sequential projection)
>At any rate, it seems (as others have noted) that the relationship
>between parallax and perceived depth is not linear.  Has anyone else 
>looked into this?

(Sorry to postpone the *really* interesting thing till here :-))
What actually means 'deeper' here? Was the nearest object, or the
window, in both categories "on-the-screen"? Then, what did the slides
have as subject matter? Many varying objects at many distances
(gradually differing) or just a few with an infinity backdrop?

This could in fact only be tested with identical subject matter, for
example only varying base (again a stereobase war?  Not for a few more
months, anyway... , so long to heroin addicts ;-).

My feeling is you *will* see more depth when deviation differs
sufficiently (say 2.4 versus 1.2 mm). Most interesting subject, Tony!

Abram Klooswyk


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End of PHOTO-3D Digest 2989
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