[photo-3d] Accommodation Convergence Link

[Gabriel Jacob <3-d@xxxxxxxxxxxx>]

Interesting thread, regarding the, "Accommodation-Convergence
(AC) link." I always like when someone questions long standing
beliefs. The belief either falls or gets reinforced, with all parties
having a better understanding of the mechanisms at play.

Now having said that, I always assumed that there was a
AC link and that it was one of the contributing factors to
headaches  when viewing 3-D.

John Rupkalvis contention that there is no link and doesn't
cause eye strain gave me (and I think a lot of us) food for
thought. Personally, I feel there is a link. If there wasn't a link,
why do people have to be "trained" to freeview? Although I
believe everyone has freeviewed even though they have not
realized it. What I mean is, the general population has already
experienced freeviewing, when they are daydreaming and
not concentrating on anything in particular. In this case, the
AC link is broken momentarily (very similar to the wallpaper
phenomenon).

I don't believe this can be used as an argument that therefore
there is no link, because when you ask the same person to
do it consciously, they can't.

An example that hits close to home for me is when I first
learned to freeview cross-eyed. I came across it rather by
accident (long story) and only when the magic eye fad came
into being did I learn to freeview parallel. For me cross-
eyed came very easily but freeviewing parallel was very hard
(not anymore though).

The problem was that I just couldn't break the physiological
barrier of looking "past" the picture. To me it was impossible
to do. The breakthrough came when I "programmed" myself
to think it isn't impossible to look past the image. Note, it was
relatively easy to do with reflective glass (in front of the image),
but this was a crutch.

Any ways, that was my personal experience and now John has
questioning our experiences/beliefs. Abram Klooswyk has tried
to shed some light regarding this that seems to add support to
John's contention. I thought to myself to try to find some
references on the Net, to try to clarify this subject and came up
with these assertions.

The Stereoskpie "Dictionary of 3-D Terms" web site,
http://www.stereoskopie.org/Glossar/DictA.htm
writes, "The physiological link which causes the eyes to change
focus as they change convergence, a link which has to be
overcome in stereo viewing since the focus remains unchanged
on the plane of the constituent flat images."

Now of course this is not the end all and final authority of
correct definitions, so I thought to look for some more
information either supporting or disputing this definition. I looked
at the well respected (?) on line Britannica encyclopedia.

Quoting the following web page,
http://www.britannica.com/bcom/eb/article/printable/7/0,5722,117507,00.html
  "The link between convergence of the eyes and focusing, or
accommodation, is very strong, and normally the two actions
work in harmony."

So here we have two references which support the claim that
there is a link between accommodation and convergence. Now
the question is, does breaking this physiological (or is it
psychological?) AC link cause eyestrain, and ultimately
headaches?

Again, citing from the same Britannica web page, "Eyestrain, or
asthenopia, is the term used to describe symptoms of fatigue
and discomfort following the use of the eyes. Although such
symptoms may result from intensive close work, particularly if
this is unaccustomed, in people with perfectly normal eyes, they
may indicate abnormalities of muscle balance or refractive
errors. Eyestrain is more likely to be manifest during periods of
fatigue or stress and is common among students working for
examinations. Refractive errors require correction and muscle
imbalance treatment. Psychological factors are often more
important than physical factors."

What does this all mean? It might mean that the abnormalities of
muscle balance (caused in breaking the AC link?) causes
discomfort and headaches, in the general population.

For me the jury is still out and I'll now cease the floor to my
esteemed colleagues! :-)

P.S. Is there a (eye) doctor in the house!?!

Gabriel, trying to focus and converge on the issue.