P3D Re: 2519 - focus (Dr. T)

[JNorman805 <JNorman805@xxxxxxx>]

	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?  Bearing in mind that I speak from a wealth of
ignorance about the physiognomy of the human eye, and freely admitting that I
could be totally off the wall on 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.  Experimentally, hold up a finger in the foreground, against a
fairly simple, easily delineated background.  When I concentrate on the
finger, the background is not blurred, but doubled.  Conversely, when I pay
attention to the background, the finger is not blurred, but doubled.  When I
close one eye and concentrate on either the finger or the background, neither
is blurred nor doubled.  I believe that the selective focus we use in flat
photography, using a larger aperture to isolate the subject of interest and
blur the rest, is a photographic convention, one of the compromises we resort
to in order to achieve an approximation of reality that is less possible in
flat photography than in stereo photography.  To me, looking at a stereo
picture (either a card or a projected slide) gives me a perception much closer
to reality in large measure because I can select whatever area of the scene is
of interest to me, foreground or background, and the rest will be doubled,
just as it is in real life.  And, just as I can in real life, I can allow my
vision to roam over the entire scene, and whatever is not being concentrated
on at any given instant is doubled (but not blurred), assuming of course that
the photo was taken with a small aperture.  I suggest, again, from a wealth of
ignorance, the hypothesis that in real life the musculature of the eye allows
subtle changes in shape that permit sharp focus even in dim light, when the
pupil widens to admit more light.  Any ophthalmologists or other experts out
there want to comment?

	On a related topic, I want to note that I'm sometimes irritated by what I
think is a certain imprecision in the terms that we all use.  As intimated
above, "focus" is one of them.  In photography, where we actually do adjust
focus of a lens, it's appropriate.  I'm not sure it's completely appropriate
to the way the eye works to bring information concerning a scene to the brain.
Also, we use "depth of field" to describe the limits of a lens under specific
circumstances to register the full range of a scene with acuity.  But we're
not really talking about the "field" here; rather we're talking about the
depth of "focus," an appropriate term to use in photography.  "Field," IMO,
should be reserved to describe the width of a scene, as I think it is in
binoculars.

	On yet another related topic, did you know that raptors have a zoom telephoto
ability?  I was in the company of some birders watching a redtailed hawk
recently, and one of them told me that you can actually see a hawk using
muscles in its neck and head to change the shape of its eyes, in order to
"zoom in" in on prey a great distance away, actually enlarging its perception
of the hapless critter that's about to become dinner.  According to the birder
I was talking to, hawks have at least three different "zoom" settings — one
for near, one for medium, and one for distance vision.  Wow! Amazing if true.
Anybody know anything about this?

	Jim Norman   


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