- Jul 2, 2009A 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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