[photo-3d] Re: again those figures!!!

[Abram Klooswyk <abram.klooswyk@xxxxxx>]

Sergio Baldissara wrote (7 Jul 2000): 
>I repeat maximum (not optimal) separation is about 
>1/30th of horizontal length (say about 2° parallax). 

Then Oleg Vorobyoff asked (13 Jul 2000):
>(...) clear up my confusion. Is there some definite 
>physical or physiological basis for the 2° maximum, or is 
>that just a guideline?  After all, most of us can easily fuse 
>the printing in a book a foot away - that is a full 12° of 
>crosseyedness (pardon the technical jargon).

Sergio again (17 Jul 2000):
>2° is intended in projection (or in enlargement) as the 
>maximal divergence of the beams from homologous points. 
>Trigonometrically 2° subtend 0.035, a little more than 1/30 
>of overall width.

This maybe is clear to Oleg, but I believe confusion still is 
possible. Yes, a somewhat gratuitous remark, which could 
apply always and everywhere :-), and language or formulation
difficulties on this international list will contribute to
the confusion, but we have to cope with that.

The word "divergence" is especially confusing.
In normal everyday vision, when you look at a distant point 
your visual axes are parallel, the angle of eye convergence is 
zero. In looking at a point at some 2 meter (about 7 feet) 
distance, your eye convergence will be about 2°. 
So the swing from infinity to two meter, or the _difference_ 
in convergence angle between 2 meter and infinity, is about 
two degrees.

In normal vision there is of course no problem in looking at 
a closer point, say at one foot (30 cm) distance, even when at 
the same time very distant objects are in your field of view.
You will get no headache or eye strain.

Not so in stereoviewing, and especially not in viewing stereo 
projection. The more or less accepted general guideline is to 
limit the depth in stereo scenes to some two degrees.

When infinity objects (or any object beyond, say, 20 meter) 
are on your pictures, don't have any object closer than 2 
meter, the rule says. That gives a _difference_ in angles of 
convergence of two degrees in viewing.

But when your farthest object is at 2 meter, the same rule of 
two degrees swing permits a closest object at one meter 
distance. Note: this all with a "normal" stereobase of about 
6 to 7 cm  (2.5").

In cross-eyed freeviewing with about 12 degrees convergence, 
the _difference_ of convergence angles, in looking at the far 
points or looking at the near points of the scene, will be 
again about two degrees, when the stereopicture is made 
according the "rule".

Oleg:
>Is there some definite physical or physiological basis for 
>the 2° maximum, or is that just a guideline?  

It is just a guideline, in so called "double depth" mounting 
the window is brought forward to about 3.5 feet, and the 
convergence difference between near point and far point will 
be 4 degrees. This does no harm, but it is not recommended to 
do this for a long series of projected slides.

In slides meant for stereoscope viewing only, the limit can be 
stretched even further, especially when there is a gradual 
increase in depth in the scene.

So this is all about _differences_ in convergence angles,
and no divergence.

Abram Klooswyk