Re: 3-D phenomenon

[P3D Jim Crowell <crowell@xxxxxxxxxxxxxxxxx>]

At 7:25 PM 6/7/96, P3D Gregory J. Wageman wrote:

>Jim Crowell writes:
>
>>Actually, that's not quite right; when you look at objects at
>>different distances in a stereo view, the convergence angle of
>>your eyes changes but the focal length of your eyes' lenses
>>(your accomodation) doesn't.  It's generally believed that
>>aoccomodation is only useful as a depth cue out to about 2
>>meters, though, & even there it's pretty weak.
>
>Are you certain that it (accomodation) *never* occurs, or is it
>that it doesn't *normally* occur because the "typical" realist slide
>(if there is such a thing) has most of its subject of interest beyond
>the distance where such accomodation is necessary?  Considering that
>the window is at about 2 meters this would be consistent, no?
>

What I said wasn't quite right.  When you're viewing a stereo
view, accomodation _shouldn't_ occur, because the entire field
of view is at the same optical distance (the optical distance
from your eye to the image surface).  However, because
accomodation & convergence are linked, some accomodation may
occur as you converge & diverge your eyes.  If this happens, it
will tend to take the image out of focus, i.e. it will hurt
rather than help.

Apparently, this can happen even in older people who don't have
much accomodation normally, i.e. convergence can drive
accomodation somewhat even when the visual stimulus can't.

-Jim C.


------------------------------------------------
Jim Crowell
School of Optometry
360 Minor Hall
University of California
Berkeley, CA 94720-2020
(510) 642-7679
jim@xxxxxxxxxxxxxxxxx
http://john.berkeley.edu/IndividualPages/jim.html



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