Not wide-angle but still distorts

Discussion in 'Digital Photography' started by Charles Packer, Apr 7, 2008.

  1. 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, Apr 7, 2008
    #1
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  2. 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, Apr 8, 2008
    #2
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  3. 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.
     
    Don Stauffer in Minnesota, Apr 8, 2008
    #3
  4. 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, Apr 9, 2008
    #4
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