Re: [photo-3d] Re: focus and convergence

["John A. Rupkalvis" <stereoscope@xxxxxxxxxxxxx>]

Some very good points.  As you all probably have gathered from my numerous
messages on the subject, I would certainly agree that focus and convergence
are associated, but not linked, at least not in the sense that gears are
linked.  I maintain that we could not view stereoscopic images on a flat
plane, either freeview or with viewing aids, if focus and convergence were
actually linked.

Incidentally, regarding your comment about myopia, a friend of mine who is
presbyopic finds parallel freeviewing more comfortable when wearing reading
glasses, but little difference (in terms of comfort, not focus) with
crosseye viewing.  Some food for thought.

JR

----- Original Message -----
From: "Abram Klooswyk" <abram.klooswyk@xxxxxx>
To: "PHOTO-3D" <photo-3d@xxxxxxxxxxx>
Sent: Saturday, October 14, 2000 4:16 PM
Subject: [photo-3d] Re: focus and convergence


> In answer to ron labbe October 13, 2000:
> > No matter how perfect a stereo projection is, the audience
> > members must disengage their normally linked focus and
> > convergence mechanisms,
> John A. Rupkalvis wrote 13 Oct 2000:
> >Focus and convergence functions of the human eyes are
> >not normally linked, although they most often are used
> >in a manner that might suggest that.
>
> When Masters speak I fear to rush in, but I believe the issue
> is a little more complicated.
>
> In France and Germany, and later elsewhere, has been said
> that the depth zone in stereoviewing (I prefer the concept of
> stereo viewing space) should be limited because convergence
> and accommodation are linked. For many years in the first
> half of this century (this still is the 20th, for another 11
> weeks) many stereoscopists believed that convergence and
> accommodation were linked as toothed gears. Therefore, in
> their opinion, the viewing depth range should be limited by
> the depth of focus of the human eye, with they measured at
> about 70 min of arch (expressed as convergence difference),
> and from there the 1 in 50 rule emerged (indeed one-in-fifty,
> not one-in-thirty, which holds for the 5P system).
>
> [George Themelis wrote Oct 14, 2000,  "... still has to
> decouple accommodation (focus) and convergence (...)
> convergence changes while the accommodation is fixed. This
> is true anytime we look at stereo slides through a viewer."
> This is in a strict sense not quite true, because
> accommodation
> can change a little while keeping clear images, because of
> the eye's depth of focus, which is rather comparable to
> lenses in general, and also depending on pupil size]
>
> About the accommodation-convergence link, the funny thing
> was that the Dutch ophthalmologist Donders already had shown
> in the 19th century how loose the coupling in fact was.
> One of his pupils wrote a thesis on the subject, which most
> people would find hard to read, not because it was in Dutch,
> but because it was in Latin...
> In his famous Treatise on Physiological Optics, Hermann [von]
> Helmholtz has quoted Donders' results.
>
> In recent years much work has been done on accommodative
> convergence and the reverse, convergence accommodation.
> 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.
>
> For viewing this means that the possibly range in practice is
> mostly more than can be achieved by the stereocamera,
> because of its limited depth of focus.
>
> There is another point of view to look at all this.
> The function of accommodation is to provide sharp retinal
> images, and the function of convergence to avoid double
> images, that is to provide single vision.
>
> It should not be surprising that both functions prefer to
> continue to do what they are invented for in the first place:
> provide sharp respectively single images, and that they
> easily give up their coupling when necessary.
> And it is necessary in stereoviewing.
>
> 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...).
>
> Abram Klooswyk
>
>
>
>