In article <4tugt9$kcd@newsbf02.news.aol.com>, stevet5710@aol.com says...

>why can the eye have a wide coverage (I think less than 24 or even
>20mm) yet it doens't have that "distortion" that ultra-wides do.
>How come!?

Ah, you have asked the basic question! (One that I thought I had
answered in various other posts in this thread, but maybe it is
time to draw it all together in one place.) Many feel that comparing
our vision to photographic processes and equipment is irrelevant,
since they hold the opinion that there are too many differences
between the two to make a comparison valid. I think they are wrong.
What we see, and how we see, can be explained in structural/chemical/
electrical terms, just as photography can, and the physical structure
of the eye does explain the perspective (and to some extent, the
angle of view) that we see, and it can be discussed in photographic
imaging terms. The brain perhaps serves to interpolate missing parts
in our field of view; sharpen some of the raw image material; adjust
the overall and local color balances we perceive; allow us to perceive
in one view an extreme range of tones without losing local contrast
and brilliance; adjust the number, sizes, and locations of the points
of attention in our field of view; and a few other things - but the
basics of our eye-perspective is in the eye structure. (In all of the
following, I am refering to the total view of both eyes together
[there is some overlap of coverage near the center of the field,
which provides for our binocular vision depth perception]).

I will take "distortion" first. It is a word that should be used
carefully (but often isn't in photography), since it refers to
specific things. There are several types of perspective (rectangular
[which most camera lenses approximate], cylindrical [as seen in
swing-lens panoramic camera images], spherical [otherwise known as
"fisheye"], orthographic fisheye [similar to spherical, but with
less curvature of off-center straight lines over most of the field,
but more near the edge], the "non-perspective" of Chinese paintings
and some architectural drawings [in which there is no convergence],
and possibly a few others. If the image perspective follows the rules
of the perspective type used, technically there is no "distortion",
odd as the image may appear.

You have noticed that the image made by most photo lenses that even
begin to be "wide-angle" (wider in coverage than the "normal" angle
of view of a photo lens, usually defined as being produced by a focal
length approximately equal to the diagonal of the image area) appears
different in perspective (and perspective effects) than what you see,
even though the angle covered is far smaller. (If you look carefully,
even the "normal" photo lens image is quite different in perspective
characteristics from what we see.) The reason is simple: we don't
see in rectangular perspective. (BTW, most people can see illumination
somewhat beyond 180 degrees of coverage - this is another indication
of non-rectangular perspective vision: angles of view approaching 180
degrees are impossible in rectangular perspective due to the enormous
size of the image area and/or the impractical shortness of
focal-length of the lens required to produce the image
[in addition to very serious illumination problems].)

For the key to beginning to understand what we see, a look at the
eye structure (and its suroundings) is in order. The eye is (roughly)
a sphere with a single-element lens placed somewhat below the surface
of the front, with a photo-sensitive area occupying approximately
the rear half of the interior surface of the sphere (photo sensitivity
in this area is not uniform - greater color and sharpness capability
is concentrated near the center, with greater low-light sensitivity
appearing off-center). The lens is somewhat forward of the front
edge of the light sensitive hemispherical area. The eye sits in a
facial surround (eyebrows, cheeks, eye-lids, eye-lashes, etc.) which,
combined with the characteristics of the eye structure, limit the
field of the two eyes combined to an illuminated view of about
180-200 degrees horizontal by about 110-120 degrees vertical (this
varies somewhat among individuals, depending on the particulars of
their physical structures) - with a rounded and softened field edge.
Within that field, sharpness and color perception is not uniform,
with excellent sharpness possible only in a very small central area,
though useable sharpness and color sensitivity may extend out to about
160-180 degrees. Most peoples' attention is concentrated in a VERY
small area near the center of the field, with some of the remaining
field surrounding the center being used for (unattended) context
for the center, and with the area near the edge of the field being
used only to perceive light and motion. Given the above, it is
possible to construct an approximation, in photographic terms,
of what we see.

In the very small central area (normal attended area), we see a sharp,
full-color image that may be thought of as approximating the view of
a VERY long telephoto lens (maybe 5000mm for a 35mm camera). (Here it
should be noted that over a narrow angle, all perspective types look
virtually identical.) Surrounding this is a very noticeably less sharp
area (still with excellent color) which is much less attended, and
which serves as context for the central image (the angle of view of
this area [but not the perspective!] approximates that of a 20-28mm
lens for the 35mm format for many people [or a somewhat shorter or
longer lens for some people]). The outside area, extending out to as
much as 200-220 degrees horizontal (yes, it is possible to see
somewhat behind the plane of our eyes!), approximates the view of
a 6-8mm fisheye on a 35mm camera (with much of the top, and some of
the bottom cropped out to form an approximately oval soft-edged
image). Awareness of (and in) this outer area for most people is
limited to motion and light perception. Color sensitivity and
sharpness are poor near the edge of the field of vision.

If we go back and look at the structure of the eye, it is possible
to get an idea of the characteristics of the perspective that we
see with (but often do not recognize as the perspective of vision).
A single-element lens projects an image of roughly 200 degrees
of subject coverage onto a hemispherically-shaped "film plane".
The rectangular-type perspective of a simple lens, projected onto
a hemispherical surface (to use any other shape would require a
vast "film plane" to encompass the image, and illumination would
fall off rapidly toward the edges) must produce a near-spherical
type of image perspective.

If this is true, and if the brain does not "correct" the spherical
(fisheye) perspective back into rectangular perspective, what are
the characteristics (and advantages) of spherical perspective that
we should see? One is that rounded objects at normal distances
(like people and cars) are not "squashed" when moved away from
the center of vision (as they are in even moderately wide-angle
photographs). Another is that, over the more limited vertical angle
that we see, subject verticals at normal distances (like trees and
buildings) remain approximately vertical and parallel with each
other when we look somewhat up and down (unlike what happens when
tilting even moderately wide camera lenses). A third is that
near-to-far subject proportions do not look exaggerated (as they
can appear to be in many photographic wide angle images). These are
characterists common to both cropped fisheye camera images and our
vision. The one bugaboo of photographic fisheye images, though, is
the curvature of off-center straight lines. This is most apparent
when the subject is abnormally close to the lens, (causing the
subject to occupy a large part of the field), or when a long straight
line in the subject (like a horizon line) crosses a large proportion
of the image field off center. In vision, our limited vertical angle
of view (which limits the range of curvature), our reduced field-edge
resolution (which helps limit the range of visible curvature and
other fisheye image edge effects), and our tendency to attend to
only a narrow angle of view at the center of our field of vision
(where all perspective types are similar) help to conceal the most
obvious differences between spherical perspective and the familiar
rectangular perspective produced by "normal" (and slightly longer)
photographic lenses.

With a little training, the curvature of long off-center straight
subject lines extending across most of the field of vision can be
clearly seen, thus ending any misunderstanding about what type of
perspective we use in seeing. If you have little or no experience
with attention point (and area) shifting within your field of
view (without changing eye direction), you can begin by looking
10 or 15 degrees away from something that is very easy for you
to see. Without moving your eyes, "look" at the off-center object.
It can take a bit of practice, but do it until it is easy, and
you can shift your attention around to any point in your field
of view easily. (Some people are able to divide their attention,
and attend to multiple points within the field of view
simultaneously.) Once you have mastered that, choose a subject
in your field of view and adjust the size of the attended area
around that subject (it can be any size, and eventually can include
the whole field of view at once [Dangerous, but fun while driving!
And you will never put up with narrow-angle "lineless bifocal"
type glasses again!]). Once you can attend uniformly to most of
your field of view, it is easy to find subjects that reveal the
curvature of fisheye perspective (looking up at a bedroom ceiling
from the bed; sitting on a pillared and roofed porch in the evening;
sitting at a desk, facing a wall with bookshelves whose horizontal
lines converge both left and right when looking straight forward;
looking across a hall, and seeing the same double convergence
(which means that the wall-ceiling/wall-floor lines must break
or bend - I vote for bend ;-); ETC! (Our whole view is composed
of curved lines [except for subject straight lines going through
the center of view, or segments that are too short to show the
curvature], just as with fisheye lenses!).
Hope This Helps