P3D Re: 2519 - focus (Dr. T)

["Dr. George A. Themelis" <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

Jim Norman asks:

>In P3D digest 2519, Dr. T writes: "in real life the background is out of
>focus while when viewing a stereo picture it is not.  Does not seem to 
>bother me."

>Are we certain about this?  ...  my observations indicate to me the
>following: When we "focus on" (perhaps "pay attention to" would be a better
>way to put it) a foreground element in a scene in "real life," the background
>is not "out of focus" (i.e., "blurred"), but rather our visual perception of
>it is doubled.  

I was repeating an indirect statement made by someone else but, yes, I am 
certain about this.  When we "pay attention to" something, the eyes 
perform two functions:  First they converge at this something, second 
they accommodate (focus on it).  

The eye has a lens in it. Muscles change the shape if this lens so that
the image is focused in our retina (equivalent to film in camera).
The image can and will be blurred if the lens of the eye has not
changed its shape to focus it.  (Unlike a camera where the lens is moved 
back and forth to focus, the eye lens changes shape to focus.)

Have you ever crossed your eyes?  You can see a double blurred image
right at the center of attention and be able to examine it.  
Accommodation and convergenced are normally coupled.  By crossing 
your eyes you are seeing double because you are not converging at 
your main object and at the same time you are seeing it blurred 
because you are focused at the point of convergence instead of the
object.  With a little practice, you can make the object clear
and still have it doubled.  That's what you do when you freeview.

Even though accommodation and convergence are coupled in real-life
seeing, they don't have to be so.  The proof of this is freeviewing.
When we freeview, we converge far away (for parallel viewing)
or close (for cross-viewing) and still manage to accommodate and
see a sharp object (right at the plane of our images).

Most stereoscopes have lenses and let the the muscles of your eyes 
relax (as if looking at infinity) so that they are focused
at the plane of the image.  They also direct the left eye at
the left image and the right eye at the right image.  The images
are sharp.  The lens of the eye does not have to work.  It can
relax at infinity.  Convergence changes as our eyes scan the image
seen via the stereoscope.  Any time we look at something close-by
in the picture, it appears clear and sharp.  The background appears
sharp but double.  This is unlike natural vision.  In natural vision
the background is doubled and blurred.  This does not bother me 
(and others) and I don't notice it.  Some people (like William
Carter) appear to have a problem with this.  (I wonder if William
can freeview or not...)

>When I close one eye and concentrate on either the finger or the 
>background, neither is blurred nor doubled.  

Of course it is blurred!  Try this again.  Look right at your finger
close-by.  Examine the background around the finger, without taking
your eye away from the finger.  The background is blurred. 

To learn more about the eye, I recommend the older book (1966) "Light 
and Vision" of the Life Science Library, occassionally found for sale
for $1! It even has stereopairs to be viewed with a single mirror.

-- George Themelis


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