Re: Poll on *Really* Wide Angle Lenses


"Chris Brown" <cpbrown@xxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:esu0t2-i6d.ln1@xxxxxxxxxxxxxxxxxxxxxxx
> In article <1123894671.317390.227450@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
> BC <brianc1959@xxxxxxx> wrote:
[ . . . ]
>
>>Also, regarding your train track experiment,
>>if you look straight down (perpendicular) at a set of straight railroad
>>tracks they do not appear to converge,
>
> Not only is this not true - because the eye has a 180 degrees field of
> view,

More correctly, *both* eyes working together cover 180 degrees (horizontally
only). Neither eye by itself comes close to 180 degrees. You can easily
prove this to yourself by closing one eye.


> it's also physically impossible. Draw a diagram - as the track tends to
> infinity (the horizon will do), the angle tended by the photons reaching
> the
> observer from each rail will tend to zero, i.e. the lines really do appear
> to meet.

If you change the angle of view so that you're looking down the track into
the distance, yes. There never was any question about that. Draw a diagram
looking straight down (which was the condition of your experiment, as I
understood it) and there is no convergence whatever. You then simply have a
map showing the railroad track--and the rails never meet.


[ . . . ]
>
>>nor to they appear to be curved.
>>If this is not your experience then you really do have an abnormal
>>visual system! Its only when you look at an angle to the tracks that
>>they appear to converge.
>
> I'm intrigued by what you think should happen as you turn your head in
> your
> worldview. Let's say you're looking down at the tracks, and you slowly
> turn
> your head back to vertical, all the time while looking at the tracks.

Which you can only see clearly over a very limited angle of view . . .


> Do
> distant sleepers slowly shrink in size as you lift your head,

You can't *see* your "distant sleepers" clearly enough looking straight down
to perceive any change in size when you start lifting your head. But as you
lift your head (I presume this means you're facing in the direction of the
tracks, rather than perpendicular to them--which means you have an even
*narrower* angle of view, since the eye's field of vision is much less in
the vertical) you change your angle of view such that you do see the
apparent convergence. And yes, the more you raise your angle of view the
more obvious becomes the convergence.

This is just the same as would be the case with a rectilinear lens (which
you seem to object to). As you tilt the lens axis away from the
perpendicular, convergence appears, and increases as it moves toward the
parallel. It is also just the same as would be the case if you drew the
railroad tracks accurately in perspective. Viewed and drawn from exactly the
perpendicular there is no convergence, no matter how wide the field of view.
Drawn from any angle other than the perpendicular, parallel lines in the
subject converge.


> as they would
> appear to in the (rather disconcerting) effect you see when a wide-angle
> lens is panned in movies,

The same effect by the way is seen in three-dimensional computer games which
have a fairly wide field of view. That may be "rather disconcerting" to you
but it's perfectly correct from the standpoint of perspective.

Human vision is not like a wide-angle lens in the movies (or anywhere else).
Why you keep trying to relate these two quite different things I do not
know.

But yes, to answer your question, convergence gradually appears as you move
away from a perpendicular view. And gradually disappears as you move back to
the perpendicular, at which point it is all gone. And gradually reappears in
the other direction as you continue to move past the perpendicular. And so
on.


> or do they suddenly go *pop* when your axis of
> view moves away from the normal?

No, it's entirely popless. Unless of course your axis of view moves with a
pop.


> I'm glad we don't actually see the world in
> the way you seem to think we do - the first effect would probably induce
> some sort of motion sickness, the second would probably be like being on
> acid (objects going *pop* *pop *pop* all the time in your peripheral
> vision
> would be seriously strange).

It's your argument that is seriously strange, if it is serious at all.

N.


.

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