Reading Glasses Strength

GlassesI have two pairs of reading glasses, one for reading and one for the computer screen.  Reading glasses are sold by their strength as measured in diopters, but I have no idea what mine are.  All I want is another pair of the same strength.  It’s a common problem for those of us who get our glasses from a rack at the dollar store.

Here is how to find out.  A diopter is one divided by the focal length in meters.  A focal length of one meter is one diopter.  A stronger lens with a focal length of half a meter is two diopters.  Simple.

You can find the focal length by going outside on a sunny day and focusing the sun with your existing glasses on a flat surface facing the sun.  The focal length will be when the image of the sun is the smallest and clear.  The light source has to be far away, so a ceiling light will not work. Most tape measures now include a metric scale, but if yours does not, a meter is a little less than 40 inches.  The focal length of your reading glasses will almost certainly be less than a meter.

RWalck@Verizon.net

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About Roger Walck

My reasons for writing this blog are spelled out in the posting of 10/1/2012, Montaigne's Essays. They are probably not what you think.
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