Marilyn Displays her Myopia about Vision Testing

Marilyn is Wrong Copyright © 1999-2006 Herb Weiner. All rights reserved.

Ask Marilyn ® by Marilyn vos Savant is a column in Parade Magazine, published by PARADE, 711 Third Avenue, New York, NY 10017, USA. According to Parade, Marilyn vos Savant is listed in the "Guinness Book of World Records Hall of Fame" for "Highest IQ."

In her <I>Parade Magazine</I> column of November 26, 2006, Marilyn displays her myopia about vision testing when she attempts to explain how the motor vehicle department can measure distance vision using an enclosure only 1 foot deep.

Sorry, Marilyn, this involves more than "foreshortening an image"

Charlie Kluepfel wrote:

A reader who uses reading glasses, but doesn't need glasses for distant vision, asks how the motor vehicle department can measure his distance vision using an enclosure only 1 foot deep.

Marilyn says this "can be accomplished by foreshortening an image, which reduces its size. For example, to learn whether you can see a basketball far away, one might determine whether you can see a penny nearby."

That's ridiculous. To the reader who asked the question, he may very well be able to see a basketball clearly at the distance, and see its seam lines, while the penny a mere foot away looks all fuzzy to him. The actual answer is that the DMV would need lenses where the subject looks into the box, with a focal length of 1 foot for a 1-foot-deep box. That makes the rays of light from each point on the target parallel, simulating what a photographer would say is an object at infinity, or in ordinary parlance, very far away. The subject would not wear his own reading glasses as they may be stronger or weaker than the strength that's equivalent to a focal length of 1 foot, and therefore not be a predictor of unaided distant vision.

By the way, Marilyn gives a misleading idea of what foreshortening involves. The Random House Unabridged Dictionary defines "foreshorten" as "to reduce or distort (parts of a represented object that are not parallel to the picture plane) in order to convey the illusion of three-dimensional space as perceived by the human eye: often done according to the rules of perspective." While part of this may involve distant objects being shrunk more than nearby ones, the major portion of the process indeed involves parts "not parallel to the picture plane". For example, the view of a square table top could have the far edge smaller than the near edge; but the side edges would be distorted even more, and appear even shorter than the far edge--that is the true foreshortening, the result of being seen lengthwise rather than directly from the side.

Charlie Kluepfel
Bloomfield, NJ last updated November 26, 2006 by

Page last modified by Herb Weiner Sun Oct 14/2007 02:11