To select lenses for the Myopter or reading glasses (close-up lenses) you must know your distance refraction, the information on the prescription for your distance glasses (not contact lenses). Sometimes only a spherical correction is given. Other times, both a spherical and an astigmatic correction are given. On occasion, there may be only an astigmatic correction. The general rule is to select close-up lenses that are 3 diopters more positive (or less negative) than your distance prescription. This is called a three diopter "add" and is the same terminology used on bifocals to indicate the power of the lower half in relation to the top half. This brings the far point in to 1/3 meter (33cm), or about 13 inches, which is a typical reading distance. Anything beyond that point is blurred. This is called reading at the far point. It is when the book is held at this distance and then pushed just a little further away, so that the print is slightly blurred, that the maximum relaxing effect on the ciliary muscles of the eyes is achieved.
Small amounts of astigmatism are usually ignored, allowing the use of spherical lenses without a cylindrical correction. Astigmatism can be thought of as a refractive error that only affects half of the cornea or lens, so that is no longer spherical. To account for the astigmatism, convert it to the spherical equivalent by cutting the amount in half. Thus, if the prescription shows -2.00 D with an additional -1.00 D in astigmatism, this could be converted to -2.50 D as the spherical equivalent. The close-up lenses would thus be +0.50 D. Quarter diopter steps such as 0.25 or 0.75 are moved to the closest half diopter value, usually in the positive direction.
Ideally, if the eyes are different, each eye is calculated separately. But that would mean that you need a prescription, since store-bought glasses have the same power for each eye. To avoid this problem, if the difference between the eyes is not great, base the close-up lens selection on the better eye. The other eye will see a little more blur, but that is of no consequence. Here are some examples of distance prescriptions with the usual close-up lenses shown in parentheses.
-4.00 x -1.00 at 90 degrees (-1.5). The spherical equivalent is -4.50.
-3.00 (zero or plano. Just use no glasses)
-2.00 x -.50 at 100 degrees (+0.75). The spherical equivalent is -2.25.
Emmetropia or normal 20/20 vision (+3.00)
The "x" on a prescription means "coupled with". That means that the spherical component and the astigmatic component which follows it are ground on the same lens. Usually, perhaps 90% of the time, the cylindrical correction is written in minus form as we have shown above. It can also be written in plus form, and some people, usually older ophthalmologists, still do this. The plus form can be easily transposed into the minus form, which is the preferred way. Ask your eye doctor to provide your prescription in minus form.
If you want to fully correct the astigmatism while applying a +3 add to the spherical correction, here is an example:
If your close work is done at a greater distance than 33cm, you would need less plus. For example, if you read at 40cm, you would use a +2.50 add. And if you read at 50cm, you would use a +2.00 add. Since a lens with a one meter (100cm) focal length is defined as being one diopter, here is the formula:
Note that the above rules do not pertain to a child who is hyperopic (farsighted). A child who is +2 or +3, for example, does not normally need reading glasses unless prolonged close work causes that hyperopia to diminish to zero. Therefore you should ask the eye doctor to tell you what the actual refractive status is, either plus or minus, and how much.
The rules for selecting close-up lenses are provided here for the use of anyone, eye doctors and non-doctors. If you need assistance in understanding these rules, contact us by email.