[photo-3d] Re: Chromatic aberrations and Chromostereopsis

[abram.klooswyk@xxxxxx]

In this thread the effects of aberration by stereoscope lenses 
and by the optical system of the eye could better be separated, 
when possible.

Gabriel Jacob 2 Feb 2001:
>The detriment of chromatic aberrations is not only
>simply seeing extra colors (blue/orange fringing),
>but also severely impacting on the 3-D effect.

George  Themelis 05 Feb 2001:
>Exactly why and how does the chromatic aberration
>impacts the 3-D effect?
    and later:
>Why do different colors appear in different
>planes?  This really translates to saying that they
>appear displaced as seen by the right and left eye.

Jim Crowell explained chromostereopsis and
John A. Rupkalvis wrote:
>with many viewers, especially card viewers, the eyes are not
>looking through the exact center of the lenses, but somewhat
>off-axis.

Chromatic aberration of simple viewer lenses does not only
affect red and blue, but of course the whole spectrum, so that
a sharp edge is blurred. We know that blur affects
stereovision.

Than Gabriel posted his chromagifs, and reported later:
>>
I checked the Internet and found most sites mentioning that
people will see the red closer. A few however did also mention
that some people will see the opposite (blue closer). They
didn't shed any more information on why and what percentage.
There was also some confusion or ambiguities on some of them.
What I could figure out was the red/blue apparent  distance
manifests itself in different ways.<<

A color depth illusion has been noticed in the 19th century, 
by Goethe and Brewster.
The Dutch physician Willem Einthoven (1860-1927, in 1924 he 
got the Nobel prize for initiating electrocardiography) wrote 
a Phd thesis (hardly 30 pages) on chromostereopsis in 1885, 
stating that transverse chromatic aberrations of the eyes 
explained the effect. Because some people see red closer and 
some not (when only red and blue are shown), and background 
colors can change the effect, there has been quite a lot of 
scientific debate on the issue ever since.

One of the points is that the optical axis of the eye (all 4 
media together) mostly does not pass through the yellow spot 
(the fovea), the point from where the line of sight 
originates. In some people it passes at the side 
of the nose through the retina, in others on the other side.
Also, the pupil is not always centered on the optical axis, and
the centering can vary with pupil size.

I found a paper by Winn et al. most convincing, summary 
below  (see PubMed at   http://www.ncbi.nlm.nih.gov/entrez/ 
Enter for example chromoster*  )
Winn B et al , Vision Research 1995 Oct;35(19):2675-2684 
"Reversals of the colour-depth illusion explained by ocular 
chromatic aberration.". [Dept of Optometry, Univ. of Bradford, 
West Yorkshire, England.]   

Abstract:
>> Although many colour-depth phenomena are predictable from 
the interocular difference in monocular chromatic diplopia 
caused by the eye's transverse chromatic aberration (TCA), 
several reports in the literature suggest that other factors 
may also be involved. To test the adequacy of the optical 
model under a variety of conditions, we have determined 
experimentally the effects of background colour on perceived 
monocular chromatic diplopia and perceived depth 
(chromostereopsis). A Macintosh colour monitor was used to 
present red, blue, and green test stimuli which were viewed 
monocularly or binocularly (haploscopically) through 1.78 mm 
artificial pupils. These apertures were displaced nasally and 
temporally from the visual axis under controlled conditions to 
induce a variable degree of TCA. Monocular chromatic diplopia 
and binocular chromostereopsis were measured for red and blue 
targets, and also for red and green targets, presented on 
either a black background or on a background which was 
composed of the sum of the targets' spectral composition (e.g. 
red and blue presented on magenta; red and green presented on 
yellow). In all cases, chromatic diplopia and chromostereopsis 
were found experimentally to reverse in sign with this change 
in background. Furthermore, we found that a given coloured 
target could be located in different depth planes within the 
same display when located on different background colours. 
These seemingly paradoxical results could nevertheless be 
explained by a simple model of optical TCA without the need to 
postulate additional factors or mechanisms.<<

On of the points in the article is that artifacts have been 
introduced because colors in recent times mostly have been 
presented on monitors, which have three color guns, all other 
colors are mixtures.
The issue is rather complicated when go into all details.

Abram.Klooswyk