Re: [photo-3d] Digest Number 302

[Olivier Cahen <o_cahen@xxxxxxxxxxxxxxxx>]

 Subject: Re: focus and convergence again

I do not completely agree with that John Rupkalvis wrote:
 "Stereoscopic viewing (viewer, projected, holographic, lenticular,
free-view, whatever) is dependent on this ability to focus and converge
at different distances, and millions of people do it all the time.
 
	My feeling is that viewing stereo slides in a stereoscope or in
polarised projection only involves a small discrepancy between
convergence and accommodation, as freeviewing needs a very heavy
discrepancy.

I do not anymore completely agree with that Abram Klooswyk wrote:
 "But stereoviewing is not the only example of uncoupling, another is
when myopic people take of their glasses. I can assure you that zero
accommodation and convergence at 25 cm (10 inch) does not give
eyestrain, I have done that most of my life when I took off my glasses
(I recommend myopia to freeviewing stereoscopists...)."

	I also am myopic, and since many years I had to converge and focus on
the same thing (for instance a book), as well with my glasses as without
them. But after many hours exercising to freeview and many headaches, I
still cannot now. Myopy is not enough to train for freeviewing.

But I completely agree with that Abram wrote:
 "There is a "zone of clear single binocular vision", which means that
to each value of accommodation belongs a range of possible vergence
values, and the reverse is also true. 
However, the range is rather broad. For practical purposes in
stereoscopy this means that the link can be almost ignored. Supposing a
fixed accommodation value in using a stereoscope, vergence can be
changed over several degrees. And then, as said, in addition a slight
accommodation alteration is Possible because of the eye's depth of
focus."

	The difference of focusing between the foreground and the background of
a normal stereo view is no more than half one dioptry. This is very
small as compared to the amount of convergence required to freeview
stereo postcards, currently as much as four dioptries. So I think normal
that 90% of all people can view stereo movies, and only a very small
percentage can freeview without training, and many people cannot, even
after training.