P3D Re: 2519 - focus (Dr. T)

[roberts@xxxxxxxxxxxxxxxxx (John W Roberts)]

>Date: Sat, 17 Jan 1998 15:16:36 -0700
>From: 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.  

Some very sketchy experiments show that in moderate-to-bright room lighting
(not the full daylight Greg W. describes, but still a common situation), I
can clearly distinguish the focus of selected objects at six feet and ten feet.
In other words, if I look with one eye, and sharply focus on an object six feet
away, then an object in the same line but ten feet away is noticeably out of
focus, and vice versa if I focus on the object ten feet away. I believe this
spacing could be relevant to some of the nearer stereo photographs. If the near
object is closer, the difference in focus is more pronounced.

>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.  

I think it's more to direct attention to a particular subject than to
approximate reality. It doesn't work as well in 3D, but it's less needed.

>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.  

I agree. But focus is also a factor (though to a lesser degree) in bright 
light. (Note: the optical effect of the cornea is actually greater than that
of the lens - but the cornea is not adjustable like the lens.)

>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.

Well, maybe *you* have fixed-focus eyes, but I don't. Actually, an optical
system can "focus" regardless of whether it's adjustable. I agree that it's
confusing that both the basic function of focus and adjustment to achieve
focus are sometimes referred to by the same term.

>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.

Sounds reasonable. "Depth of focus" is also applicable to human vision.

>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   

I've never heard that - it's amazing if true. If the eye is modeled as a
simple lens a fixed distance from the retina, I don't see how it could work -
you might lengthen the focal length of the lens, but then the distant objects
would go out of focus. If the cornea plays a role similar to that in human
vision, and the focal length of the lens can be adjusted, and the spacing
between lens and cornea and/or the spacing between lens and retina can somehow
be adjusted slightly, would that be enough to implement some kind of workable
long-distance zoom??? "Muscles in head and neck" would indicate external
squeezing of the eyeballs, rather than internal adjustment - can that shift the
spacing of optical elements and retina to a significant degree?

I have heard that daylight-hunting raptors tend to have much greater visual
acuity than typical humans - more receptors on the retina per steradian of
the field of view. With that, and with sufficient processing power, they
wouldn't so much *need* optical zoom capability to see a small object at a
great distance.

John R


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End of PHOTO-3D Digest 2522
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