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7966Judging size

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  • Joseph Morlan
    Jul 2, 2009
      A colleague suggested I post this to the group.

      There is a phenomenon in size illusion where a rear object appears larger
      than a front object of the same size when seen through binoculars, scope or
      long camera lens. The actual foreshortening caused by the vanishing point
      perspective doesn't change, but the magnification makes everything appear
      closer so relative sizes become distorted. The illusion is apparent in
      life and in photos.

      Try viewing a rectangular piece of paper edge-on with binoculars. Put the
      paper on a table and step back until you can get the paper in focus in your
      binoculars. If you do it right, the paper will appear to be expanded
      towards the back, like a trapezoid.

      The effect is most evident when the paper is close to you, because the
      amount of expected foreshortening of the back object is greater under that
      geometry. The actual foreshortening is less, resulting in the illusion
      that the back object (bird) is larger than the front. This is why even
      relative size can be difficult or impossible to judge, even when two birds
      are seen or photographed together for direct comparison.

      The late Peter Grant wrote a paper on the subject in "British Birds" many
      years ago and I was one of his study participants.


      Grant, P. J. 1983. Size illusion in birds. Br. Birds 76:327–334.

      Joseph Morlan, Pacifica, CA jmorlan (at) ccsf.edu
      SF Birding Classes start Sept. 15 http://fog.ccsf.edu/~jmorlan/
      California Bird Records Committee http://www.californiabirds.org/
      Western Field Ornithologists http://www.westernfieldornithologists.org/
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