| CLOSE-UP LENS SELECTION |
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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. Each eye is calculated separately. Here are some examples of distance prescriptions with the usual close-up lenses shown in parentheses. -6.00 (-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. 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 refraction is, either plus or minus. The rules for selecting close-up lenses are provided here for the use of anyone, eye doctors and non-doctors. |
国内订购者，凡圆通快递可到达的地区均发免费快递，其他地区发邮政快包。海外顾客订购，均发航空包裹。 