Explanation of Diopter Strength
The optical power of a lens with a focal length of 1 meter (about 39 inches) is said to be 1 diopter. Since diopter strength is based on the reciprocal of the focal length, a 2 diopter lens is not 2 meters but 1/2 meter, a 3 diopter lens is 1/3 meter and so forth. This is important because magnification increases as the focal length gets shorter, which is why a prescription for a higher diopter correction means you need more magnification.
The optical power of the human eye is about 40 diopters. The eye of a normal young person can adjust an additional 20 diopters. By age 25 this focus flexibility is usually reduced to about 10 diopters and by age 50 to a mere 1 diopter. It's this diminishing capacity for adjustment, called presbyopia, that warrants reading glasses.
Reading glasses compensate for the loss of natural adjustment. Optical power is additive so the correction is straightforward. An optometrist prescribes a lens that increases the magnifying power of the eye, usually in steps of a quarter-diopter over a range of 1 to 3 diopters. A quarter-diopter is a large enough step that most people can self-prescribe their own reading glasses by simply trying on different diopter strength lenses and reading the example text.
We do not sell pre-fabricated glasses. Instead, each pair of our glasses is custom fitted by a certified optician. Generally, the eye care industry classifies reading glasses lenses in diopter strengths that increase by a factor of .25 (e.g., +.75; +1.00: +1.25; +1.50; +1.75; +2.00; +2.25; +2.50; +2.75; +3.00 etc.).
The easiest way to find out the diopter strength you need is to visit a drug store, department store or other retail outlet that sells cheap reading glasses, and try on a few different strengths. If you have no idea where to start, consider using this average age formula: If you are under 40 years old, you probably should start with a +1.25 strength. Between ages 40 and 45, you probably need +1.75. At ages 45 to 50, you probably need +2.00. After age 50, you probably need +2.25.
While this is based on averages, and you should try to find the precise lens that helps you, the nice thing about buying FOCUSERS is that if you make a mistake, you can quickly and at no risk to you, rectify it under our RETURN POLICY and NO EXTRA COST EXCHANGE POLICY.
In addition to the methods mentioned, you can always visit an optometrist or ophthalmologist to have your eyes refracted.