3M Virtua Sport With Foam Gasket
Diopter +2.5: Explained Below
- Certification(s): ANSI Z87.1-2010, CSA Z94.3-2007
- Lens Tint: Clear
- Lens Coating: Anti-Fog
Comfortable, lightweight frame features the Cord Control System (CCS) with an optional foam gasket attachment
CCS is designed to keep a corded earplug attached, untangled, and ready for use
Foam gasket helps limit eye exposure to nuisance dust
The optical power of a lens with a focal length of 1 meter (about 39 inches) is said to be 1 diopter. Because the formula 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 accommodation (the ability to alter focus)is usually reduced to about 10 diopters and by age 50 to a mere 1 diopter. It is 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 simple trial and error.
The level of magnification in the lenses of reading glasses is called diopter strength. A very weak magnification would be found in reading glasses with a diopter strength of +.75 or +1.00. Off-the-shelf reading glasses sold in drug stores or elsewhere on the Internet are often offered in diopter strengths that start with 1.50, and with only a few options for stronger lenses.