Diopter Strength Explained

Diopter strength refers to the optical power of a lens. It's commonly applied to reader-strength eyeglasses or bifocals, as well as telescopes & other magnifying lenses. Lenses with a focal length – defined as the distance between the center of a lens and its focal point – of 1 meter (about 39 inches) have a diopter strength of 1. Since diopter strength is based on the inverse 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 lens focal length gets shorter, which is why wearing higher diopter strength lenses allows for sharper, clearer images closer to your eyes.

Illustration of human eyes able to focus and unable to focus on text without diopter strength readers or bifocals
A: Younger, more flexible eyes can adjust to focus on both near and far objects. B: As we get older, our eyes lose flexibility and can no longer focus on near objects without readers, bifocals or prescription eyeglasses with appropriate diopter strength lenses. 

The optical power of a healthy human eye is about 40 diopters, meaning the ability to focus on an object or image 1/40 of a meter from the eye. The eye of a typical young person can adjust an additional 20 diopters. By age 25 this focusing flexibility is usually reduced to about 10 diopters. Due to natural weakening of eye muscles that focus your vision and hardening of the eye lens, by age 45 the human eye is typically a mere 1 diopter. It's this diminishing capacity for the eye to properly focus its lens – called presbyopia – that results in blurred up-close vision and warrants corrective glasses.

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Reading glasses compensate for the loss of natural eye focal adjustment. An optometrist may prescribe 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. 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.). 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, as typically found in a drug or grocery store.

Eye illustration of normal vision vs low diopter strength due to presbyopia

The easiest way to find your diopter strength is to visit a drug store, grocery store or other retail outlet that sells reading glasses, and try on a few different strengths. If you have no idea where to start, try 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, return 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 tested. Then, if you require a different diopter strength for each eye or additional prescription correction, you can send in your prescription with your Focusers order and we'll expertly prepare your frames with your lenses. Or simply order just the frame and take it to your local optometrist, who can use the plain demo lenses that come with all Focusers frame only orders as a lens template.