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Not wide-angle but still distorts

 
 
Charles Packer
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      04-07-2008
When a zoom lens of a point-and-shoot is at the unzoomed
position, typically about 38-40mm-equivalent, we expect to see
geometric distortion toward the edges. I always thought
that zooming in above 50mm. should eliminate that. But
I tried to register two images taken on successive days
from the same position, using softare and control points to
align them and discovered that there is still distortion.
The two images are
cpacker.org/a1.jpg
cpacker.org/a2.jpg
The zoom amount (35 mm equivalent value) is about 77 mm.
There is perfect alignment across the bottom because the
two control points are there -- the white plastic pipes on
the right and left sides. But -- if you can click back and
forth between them, it's apparent that the tree trunks at
the top spread out and back slightly, apparently because
in the field of view, the whole scene was positioned 400
pixels higher in one image than in the other. This is about
one sixth of the 2454 pixel height of the field of view.
In others of this series, the difference was less, and
the mis-registration is too small to be annoying. Lesson:
pay attention to camera aim more carefully from one day
to the next.

--
Charles Packer
http://cpacker.org/whatnews
mailboxATcpacker.org
 
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Charles Packer
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      04-08-2008
On Apr 7, 11:43*am, "David Ruether" <(E-Mail Removed)> wrote:
> Almost all lenses have some linear distortion, even non-zooms.
> More on this is here -www.donferrario.com/ruether/lens_distortion_types.htm,
> and this may also be interesting -www.donferrario.com/ruether/lens_perspective_types.htm.
> Zooms almost invariably have at least simple barrel distortion
> toward the wide end and pincushion distortion toward the
> long end - but things can get MUCH more complicated...


Interesting. It's a revelation to me that there is something called
a "rectangular perspective" lens. If I graduate to a digital SLR will
I be able to find one for any make of SLR that I buy?

--
Charles Packer
http://cpacker.org/whatnews
mailboxATcpacker.org
 
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Don Stauffer in Minnesota
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      04-08-2008
On Apr 7, 10:43 am, "David Ruether" <(E-Mail Removed)> wrote:
> "Charles Packer" <(E-Mail Removed)> wrote in messagenews:(E-Mail Removed)...
> > When a zoom lens of a point-and-shoot is at the unzoomed
> > position, typically about 38-40mm-equivalent, we expect to see
> > geometric distortion toward the edges. I always thought
> > that zooming in above 50mm. should eliminate that. But
> > I tried to register two images taken on successive days
> > from the same position, using softare and control points to
> > align them and discovered that there is still distortion.
> > The two images are
> > cpacker.org/a1.jpg
> > cpacker.org/a2.jpg
> > The zoom amount (35 mm equivalent value) is about 77 mm.
> > There is perfect alignment across the bottom because the
> > two control points are there -- the white plastic pipes on
> > the right and left sides. But -- if you can click back and
> > forth between them, it's apparent that the tree trunks at
> > the top spread out and back slightly, apparently because
> > in the field of view, the whole scene was positioned 400
> > pixels higher in one image than in the other. This is about
> > one sixth of the 2454 pixel height of the field of view.
> > In others of this series, the difference was less, and
> > the mis-registration is too small to be annoying. Lesson:
> > pay attention to camera aim more carefully from one day
> > to the next.
> > --
> > Charles Packer

>
> Almost all lenses have some linear distortion, even non-zooms.
> More on this is here -www.donferrario.com/ruether/lens_distortion_types.htm,
> and this may also be interesting -www.donferrario.com/ruether/lens_perspective_types.htm.
> Zooms almost invariably have at least simple barrel distortion
> toward the wide end and pincushion distortion toward the
> long end - but things can get MUCH more complicated...
>
> BUT, having just looked at your images, you have a very
> different situation from linear lens distortion. You are seeing
> the effects of perspective and tilting (as you say...), best I
> can tell. This is not "distortion"...
> --DR


As an old lens designer, this is something that has bothered me a lot-
the use of the term "distortion" to mean many things that have nothing
to do with the optics term.

At least we are now seeing fewer folks calling blur "distortion,"
something that was common in this group.
 
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Charles Packer
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      04-09-2008
On Apr 8, 10:02*am, Don Stauffer in Minnesota <(E-Mail Removed)>
wrote:

> At least we are now seeing fewer folks calling blur "distortion,"
> something that was common in this group.


I was using "distortion" the way a gardener uses "weed":
whatever I don't want. Years ago, when I first read about
low-frequency room effects in audio, I was surprised that
it was called "distortion," because up to then I had
thought of distortion as something produced only by the
equipment. Anyway, if you can give me a shorter phrase
than "effects of perspective and tilting," I'll use that.

Whatever it's called, I'm astonished that it shows up
with only a slight tilt difference between two shots of
the same scene from exactly the same location. I realize
now that this was what was dogging me last autumn in a
long daily series where I attributed the problem to wind
( http://cpacker.org/trees ).

--
Charles Packer
http://cpacker.org/whatnews
mailboxATcpacker.org
 
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