# how glasses prescriptions work

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I’ve had glasses since I was 7 and I’ve never been able to grasp how a prescription is determined. Like what does the plus and minus mean? And how does that relate to 20/20 vision? Are there two different systems?

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The “strength” of a lens (that is, how hard it affects light) is measured in a unit called the “diopter.” This unit is (bear with me, I’ll break it down)

>the reciprocal of the focal length in meters.

That is, if you have a lens with a strength of +1 diopter, that means that light that passes through it will come to a single point one meter after it passes through the lens. +2 diopters means (2^-1 = **1/2**) meter away.

A negative diopter is used for lenses that spread light apart, and you draw “virtual lines” back through the lens to arrive at the “virtual” focus point. The strength otherwise means the same thing.

This lens strength is added to the strength of the human eye to bring the light back to the retina to restore standard 20/20 vision.

Now, what does that mean? Simply put, if you have *20*/**40** vision, that means that if a person with normal eyesight can clearly see an object from **40** feet away, you have to be *20* feet from it.

The diopter system is quantitative: you can directly measure a lens’ strength and determine things about it.

The 20/20 system, by contrast, is qualitative; you can’t perfectly tell what it means to have 20/20 vision, which is why eyeglass places use that eye chart and have you read the lines of letters. Roughly speaking, each line is calibrated to be near a certain visual acuity.

Your eye is basically a light focusing system. Rays of light enter your eyes, and the combination of your cornea and your lens focus the rays onto a structure called the retina, which is located at the back of the eye. If your eye focuses light exactly on the retina, you have good vision and see things sharply. People who are farsighted or nearsighted have eyes with properties that lead to light not being focused exactly on the retina. In nearsighted people, the light focuses on a point in front of the retina, whereas in farsighted people, light focuses at a point behind the retina. In both cases, because light is not perfectly focused on the retina, objects appear blurry (‘out of focus’).

In an eye exam, the eye doctor is basically determining what kind of extra lens (glasses or contacts) is needed so that the combination of the extra lens + your actual eye will lead to light being focused exactly onto the retina, thus restoring sharp vision. The + and – of a prescription are related to whether you’re nearsighted or farsighted and basically mean whether the lens will move the point at which light rays focus forward or backward. If your natural eye causes light to focus behind the retina, you need a lens that will move the focal point forward onto the retina. If your natural eye causes light to focus in front of the retina, you need a lens that will move the focal point backward onto the retina.