It is not unusual for a patient say, “My old glasses were 20/50; what are they now?” I’d like to clear up the perception that “20/something” tells what the prescription is.
The average vision a person can obtain with their best eyeglass correction is 20/20. This is called visual acuity. Some have better than 20/20 acuity, and some worse. If a person sees 20/40 without any glasses, that means the smallest object he can make out at 20 feet away can be seen by the average person from a distance of 40 feet. If, with his glasses, he can see 20/15, he is seeing better than the average person. The smallest object he can see at 20 feet away can only be seen at 15 feet away by the average person. The other point to draw from this is that visual acuity vision is measured for each eye.
There can be many reasons why eyes can have different best-corrected visual acuities. Some differences are caused by eye diseases or cataracts, a gradual and natural loss of transparency of the lens of the eye. Some result from lack of development, such as an eye turn a person is born with. Some people have nothing wrong with their eyes; they simply have visual acuity that is on the lower end of the 20/20 average.
So, how do we describe the actual strength of the glasses? The strength required to give each eye its best visual acuity is measured in a unit called a diopter. The dioptric power of a lens describes how close the lens focuses light rays coming through it. The higher the number of diopters, the shorter the focal length is.
Now for your math lesson: The focal length is the inverse of the diopter strength in meters. Remember the metric system? That’s the logical measurement system we Americans have refused to adopt, favoring instead a system based on King Henry VIII’s shoe size. So, a four-diopter lens has a focal length of one-quarter meter, or about nine inches. A one quarter-diopter lens has a focal length of four meters, or about 12 feet.
For a person who is nearsighted and can’t see distances, the degree of -4.00 starts losing clarity outside of nine inches away. His eyes have too much focusing strength. The -4.00 lens will fix that patient’s problem by decreasing the focusing strength until the eye can focus at what we call optical infinity. At that point, the person will see clearly at long distances. A farsighted person of +4.00 requires a 4.00 D lens to increase his focusing strength to see at optical infinity.
Getting the best lens strength does not tell us what a patient can see with that lens. Hopefully the patient will see 20/20 or better. But if they can’t, it’s then the job of the eye doctor to determine what is causing any diminished visual acuity.
Dr. David Eilbert is an optometrist at North Fork Optical in Mattituck.