Marilyn didn't tell you about Contact Lenses

Marilyn is Wrong Copyright © 1998 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 Parade Magazine column of July 19, 1998, Marilyn claimed that compared to glasses, "the view through contact lenses is usually much more accurate, because they correct your peripheral vision."

Angular Magnification

D. Alan Chandler MD (Ophthalmologist) <> sent the following response:

Marilyn gave a "correct but incomplete" answer to the question regarding the visual distortion provided by contact lenses vs. glasses. I agree that the woman in the question will appear more realistic to herself when using contact lenses rather than with glasses, but this has nothing to do with peripheral vision, as Marilyn stated in her response.

Image distortion caused by an optical system such as glasses and contact lenses is due to a concept known as "angular magnification". Angular magnification is defined as "(the visual angle subtended by the image produced by the system) divided by (the visual angle subtended by the object without the system). An eye with refractive error (nearsighted or farsighted) can be though of as a normal eye with an "error lens" added at the position of the crystalline lens of the eye. An additional lens then placed either 5mm anterior to the crystalline lens on the cornea (a contact lens), or placed about 17mm anterior to the crystalline lens in a glasses frame will then correct the refractive error introduced by the "error lens". For example, an eye with no crystalline lens will require a glasses lens of about +12.00 diopters to correct the vision. The same eye will require a contact lens power of +14.00 diopters (being 12mm closer to the eye), and an intraocular lens power of about +15.00 diopters (if the intraocular lens is placed 5mm inside the cornea). Thus, the eye is deficient in power by -15.00 diopters.

This in effect sets up a telescopic lens system (a Galilean telescope), with the magnification being equal to the ratio of the lenses used. For the glasses situation, the image magnification equals 15/12, or 25%. For the contact lens system, it is 15/14, or about 7%. Thus, in this scenario, the glasses will actually cause more image magnification as compared to the contact lenses.

However, this situation is reversed with negative powered glasses and contact lens powers (myopia). Here, the glasses will cause a reduction in image size, but the contact lens will still give a more realistic image size, since the contact is closer to the actual lens of the eye.

Probably the woman in Marilyn's question actually has astigmatism, which leads to more image distortion than a simple nearsightedness or farsightedness correction. This is because astigmatism correction gives power along a certain axis, measured from 1 to 180 degrees. Glasses will still give more distortion than contacts as above, but the image may appear foreshortened or elongated, depending on the astigmatism axis.

Nevertheless, peripheral vision is not involved in this problem, only the central vision. last updated July 25, 1998 by