How to Convert a Contact Lens Prescription to a "20/20" Measurement?
April 17th, 2010 | See the Top Answers to this Question Below
my contacts say -3.00 on the box but what would that be in terms of 20/20 reading?
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The -3.00 is a diopter reading, it has to do with the focal length of the lens you’re being prescribed. In this case it means your lens will have a focal length of -1/3 of a meter (the -1/3 has to do with the fact that it’s correcting for nearsightedness). While it doesn’t directly translate to Snellen fraction (that’s the 20/20 thing), from what I gather a prescription of -3 diopter would be about equal to 20/250 or so uncorrected.
If you want to look more into it, you can search "Visual Acuity" and perhaps some information on lens such as "focal length", "diopter", etc
-3 diopters would be about 20/250. Diopters measures how much the lens bends the light going into your eye and 20/20 measures how well you see the specially designed letters and numbers on a standardized eye chart at 20 feet(you are the second number – in your case 250 feet) compared to the average person.
If a person sees 20/40, at 20 feet from the chart that person can read letters that a person with 20/20 vision could read from 40 feet away. cite: http://www.mdsupport.org/library/acuity.html