LPF Diopters and 20/Something

Diopters and 20/something

Eyesight is commonly evaluated by reading a Snellen eye chart from a distance of 20 feet—the distance beyond which the lens does not need to adjust for distance. (This is the equivalent of the infinity setting on a camera.) Variances from the “ideal” 20/20 can be caused by myopia, astigmatism and other variables including the lighting in the room. 20/something provides a descriptive approximation of visual acuity.

The power of a lens placed in front of the eye to correct for myopia is measured in terms of diopters. Diopters are a prescriptive measure of myopia. If myopia is the only variable, the following approximate relationships exist between measurements in terms of diopters and in terms of 20/something:

In Diopters	20/something
-0.50	20/25 to 20/30
-1.00	20/30 to 20/50
-3.00	20/300
-4.00	20/400
-5.00	20/600
-6.00	20/800

Astigmatism can be corrected optically by using cylindrical lenses that modify the non-spherical component of the error. Again power is measured in diopters. In this case, positive and negative diopters do no cancel each other.

In Diopters	20/someting
0.5	20/25
1.0	20/40
2.0	20/70
3.0	20/100
4.0	20/200