Re: Poll on *Really* Wide Angle Lenses


"Chris Brown" <cpbrown@xxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:o29ms2-rni.ln1@xxxxxxxxxxxxxxxxxxxxxxx
> In article <1123540388.747852.150750@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
> BC <brianc1959@xxxxxxx> wrote:
>>
>>Chris Brown wrote:
>>> Try comparing to what you'd see with your *own* eye. When you look at
>>> that
>>> same brick wall, the bricks directly in front of you appear bigger than
>>> the
>>> bricks further away, or if you prefer, the lines of motar between them
>>> converge towards the vanishing point. This is just a restatement of the
>>> one of the first things everyone knows about vision - that objects look
>>> smaller the further away they are.
>>>
>>
>>That's not how perspective and vanishing points work. If you look
>>straight at a brick wall then the upper and lower vanishing points are
>>both at infinity. Its only when you look on the brick plane at an
>>inclined angle (non perpendicular) that you can begin to see vanishing
>>points and the more distant bricks start to appear smaller.
>
> You are quite simply wrong here. Objects look smaller the further away
> they
> get - it doesn't matter what angle you look at them at.

No, BC is correct here, at least as far as flat-field rectilinear
photographic lenses are concerned. The bricks (or other details on what is
essentially a two-dimensional surface) do not change in size regardless of
their position, as long as the lens axis is perpendicular to the wall.


> You can see this
> yourself - find something that nicely demonstrates vanishing point
> perspective, such as powerlines heading off to the distance, or a long
> straight road. Look straight at it and observe how things look smaller the
> further away they are.
>
> Now turn through 90 degrees, so that you are perpendicular to the scene.
> Continue to lok straight ahead, but observe your pervious subject using
> your
> peripheral vision. Notice how things still look smaller the further away
> they are.

One problem is that that's extremely difficult to do. While both eyes
working together give us about a 180-degree horizontal field of view, only
the center portion has enough sharpness and detail to give us much clear
information about the image. But I think much the larger problem is that
it's an apples and oranges comparison: The rectilinear camera lens projects
its image on a flat plane. The eye's lens does not. I don't believe there is
any way that a rectilinear camera lens can reproduce on a flat surface the
hemispherical image formed on the retina by the eye's lens. I suspect that a
fisheye lens actually comes closer to doing that, but seems to have
curvilinear distortion because we are looking at its image on a wrong (i.e.,
flat) surface at a wrong distance. The eye-brain system sorts out shapes and
sizes in three-dimensional space, in a way that I don't think any camera
lens can do.

But David J. Littleboy's argument seems convincing. As he shows, the
rectilinear lens simply must receive fewer photons from the more distant
same-sized object, so how there could *not* be light falloff is very hard to
understand, unless (as I suggested earlier) the aperture somehow becomes
progressively larger for off-axis imaging.

N.


>
> Or, if you are looking at cows in a field, the one that's 300 metres away
> is
> never going to look the same size as the one that's 3 metres away,
> regardless of the viewing angle you use to look at them.


.

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