Re: focus by chromatic aberration

[P3D John W Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Tue, 22 Oct 1996 23:56:28 -0500
>From: P3D Peter Abrahams  <telscope@xxxxxxxxxx>
>Subject: focus by chromatic aberration

>As a followup to a discussion of a week ago, here is an e-mail I received:

>I agree with the general statements below
>as well as your hypotheses as well as your respondent's take on which
>colors should be more or less in focus.  The general notion of chromatic
>aberration as a cue to accommodation is fairly widely accepted and is in
>textbooks.  The most current and compelling work (including the effects of
>doublets that reverse chromatic aberration) has been done by a fellow named
>Phil Kruger at SUNY Schnurmacher Vision Research Institute in Manhattan.
>Most of his stuff is published in the journal Optometry and Vision Science.

SUNY and Schnurmacher have web pages - unfortunately, Phil Kruger doesn't
seem to have one. Does your reference have any information on how to find
"Optometry and Vision Science" (i.e. is it put out by a particular society
or other organization?)

[Side-note: the common practice (which has been discussed recently) of
reusing terms in a different field has always been confusing to novices,
and now it's a more serious nuisance because of the limited comprehension
designed into computer programs. If you ask a search engine to look for
"accommodation" on the Web or on Usenet, you'll get several hundred thousand
hits on references to hotels, which makes it very difficult to pick out
references to the focus of the human eye. In the future, the smart researchers
who need new terminology will come up with terms containing unique strings.
If we had it to do over again, "3-D" would be better described by a new
term such as "tridionic". :-)]

>>the eye uses chromatic aberration to cue the ocular muscles for
>>increased or decreased accomodation... 
>>other work details how the  eye has difficulty focusing
>>in monochromatic light.

>>>  - for an object closer than the current focal distance of the eye, the
>>>      red is a little bit more blurry (out of focus).
>>>  - for an object further than the current focal distance of the eye, the
>>>      blue is slightly more blurry.

To put that back into concrete terms for those who missed the first part of the
discussion, suppose you're looking at a scene using one eye, and an object
you wish to look at is out of focus with the current focal setting of the eye.
To get a good view the focus of the eye must therefore be adjusted, and the
most efficient way to do this is to know whether you should move the focus
closer or further. If your eye picks up both red and blue from the object,
and the red is fuzzier than the blue, then the object is closer than the
current focal distance of the eye, and you need to adjust the focus closer.
If the blue is fuzzier than the red, then the object is further than the
current focus, and you need to adjust the focus further away. If the light
from the object is monochromatic, then chromatic aberration is not available
as a focal cue, and other methods (possibly trial and error) must be used.

Given the assurances that this focal cue is really used in human vision, I
would assume this means there's some circuit in the visual system which is
more sensitive to chromatic aberration than the perceptual circuitry, since
I generally don't seem to notice chromatic aberration in out-of-focus objects.
(It's even possible that the perceptual circuitry actually tries to suppress
conscious perception of chromatic aberration resulting from the optics of the
eye.)

It would be interesting to find out whether the research has included
focusing on black silhouettes against a lit background. I would guess that
the mechanism would work in this case too.

John R


------------------------------

End of PHOTO-3D Digest 1642
***************************
***************************
 Trouble? Send e-mail to 
 wier@xxxxxxxxxxxxxxx 
 To unsubscribe select one of the following,
 place it in the BODY of a message and send it to:
 listserv@xxxxxxxxxxxxxxx 
   unsubscribe photo-3d
   unsubscribe mc68hc11
   unsubscribe overland-trails
   unsubscribe icom
 ***************************