Nonprescription eyeglasses are primarily for people who can no longer read fine print. This condition, called presbyopia or "old vision," usually begins at about the age of 35 and progresses until about the age of 55. This inability to focus clearly at near distances is a universal condition associated with human aging. Presbyopia is not a disease, nor does it represent a degenerative process or presage blindness. Presbyopia is simplistically a result of the gradual hardening of the eye's natural lens proteins, rendering the lens less elastic and thus less able to change its shape and less able to alter its focal length to nearer objects.
Although over-the-counter (OTC) glasses do not appear to harm vision, if they are not appropriate for a person's eyes, they can cause eyestrain, or asthenopia, after prolonged use. In addition, OTC glasses have the same focusing power in both the left and the right eye, which often does not match the exact prescription of many patients who have a significant difference in the power of their eyes. This power mismatch, called anisometropia, can also cause eyestrain or asthenopia even though the OTC glasses may be correct for one eye.
Most vendors offer standard and bifocal lenses, and some vendor locations also offer transitional lenses that lighten up indoors and darken when outdoors. Reading glasses may be available in both full reading vision and in bifocals. Bifocals, invented by Benjamin Franklin in 1790, have a lower segment to see up close, with an upper segment consisting simply of clear glass for distance vision.
The quality and durability of OTC eyeglasses vary widely. Cheap models -- costing as little as a few dollars -- may have lenses that scratch easily and distort vision. Expensive models -- costing $50 or more -- may incorporate some or all of the desirable characteristics of high-quality prescription lenses. There is no permanent harm to the eyes from wearing poorly focused or distorted lenses. However, eye strain, computer vision problems, and poor-quality vision from cheap or improperly fitted eyeglasses can reduce productivity and accuracy.
Manufacturers rate the power of nonprescription glasses from weakest (+1.00 diopters) to strongest (+3.50 diopters or more). When making a selection, the customer tries on eyeglasses and reads fine print while standing 12 to 14 inches from the print, or simply holding the printed material at a comfortable, customary reading distance. The customer keeps trying on glasses with different power ratings until the print comes into sharp focus. Suppose, for example, the print comes into focus when the customer is wearing eyeglasses with a power rating of +2.00. The customer then knows that a +2.00 rating is correct and can select from styles with that rating. This same customer may prefer a lower power, such as a +1.25, for intermediate-distance work such as knitting or using a computer. Intermediate work generally focuses on a distance of 16 to 24 inches.
As we age, our eyes continue to gradually lose the ability to focus up close. Thus, a younger person first requiring reading glasses may require only +1.00 diopter lenses, which may progress with time to +2.50 diopter lenses.
Trifocals and variable lenses
Some people will require multiple distances or focal lengths for a wide variety of tasks, including near and intermediate tasks. Near tasks include reading, arts and crafts, fingernail trimming, or model airplane building, while intermediate tasks include computer use, knitting, shaving at the bathroom mirror, or reading piano music while performing, for example. When presbyopia advances, our eyes lose the ability to see intermediate as well as near items. For these special needs, a trifocal lens may be required. This allows for clear distance, intermediate, and near vision. The trifocal lens is generally placed in the same location as the bifocal, with a smaller segment closer to the center of the lens.
A further advance is the variable focus lens, in which the lines between the bifocal and the distance lens are eliminated, providing a gradual transition from near to far, with intermediate therefore in between. Varilux is a common brand name among numerous others. Many people prefer these more expensive lenses because the transition line is so annoying. It may take several weeks to become accustomed to this type of lens. These lenses provide the intermediate distance in the region between the bifocal and the distance segments, thus creating a trifocal capability using a simple subtle tilt of the head as well.
In some cases, customers may also be asked for the "pupillary" distance. This is the interpupillary distance, the distance between the pupils of the eyes. The pupils, although black in appearance, actually represent the round clear optical space defined by the center of the colored brown or blue iris. This distance is usually measured in millimeters.
Doctors do not recommend nonprescription glasses as a permanent solution to significant eye problems because they usually are not as accurate as prescription lenses. In addition, the use of OTC glasses could lead to the postponement of an eye examination that could reveal a serious underlying problem such as glaucoma, cataract, or macular degeneration. However, OTC glasses are okay to keep as "backups" for times when prescription glasses have been misplaced or are being repaired. Be certain to follow the recommendation of your eye care provider, an ophthalmologist, or an optometrist, before relying upon OTC readers.
Sunglasses that can be purchased without a prescription come in hundreds of models with varying shapes, materials, coatings, configurations, and colors.
A must-have feature for all sunglasses is complete (100%) protection against the potentially damaging effects of the sun's ultraviolet A (UVA) and ultraviolet B (UVB) rays. Avoid buying dark sunglasses that do not offer 100% UV protection. Dark glasses prompt the pupil of the eye to dilate (enlarge), allowing UV rays to enter. So, if dark glasses lack complete UV protection, they can paradoxically promote damage to the eye.
Ultraviolet rays are well-known contributing causes of the following eye conditions:
Ultraviolet rays are well-known causes of the following eye conditions:
- Macular degeneration is a potentially blinding condition in which the pigment cells in the optical center of the retina slowly degenerate during the dry form of this increasingly common disease, creating distorted vision. The rapidly changing form, or wet macular degeneration, can lead to sudden vision loss. Fortunately, remarkable new medications are now available to arrest and reverse damage caused by wet macular degeneration if caught in time.
- Cataract is an extremely common condition in which the clear crystalline lens of the eye develops progressive clouding and opacity. People living at high altitudes, in equatorial regions, and those with outdoor occupations generally develop cataracts at a younger age than others due to increased lifetime UV exposure.
- Pterygium is a degenerative process on the ocular surface in which the normally clear mucous membrane, called the conjunctiva, overlying the white part of the eye, or sclera, gradually becomes cloudy and elevated, and then grows toward the center of the cornea. Advanced cases can lead to vision distortion and permanent loss of visual acuity. Once again, people from equatorial regions and those with outdoor occupations are much more likely to develop pterygium. Fishermen, farmers, sailors, surfers, construction workers, and sun worshippers are particularly susceptible.
- Basal cell carcinoma is a slow-growing superficial cancer in which abnormal skin cells turn cancerous, particularly on the lower eyelids, where there is more UV exposure than the upper eyelids. When detected early, basal cell carcinoma of the eyelid can be cured with surgical excision.
- Squamous cell carcinoma is a more rapidly growing form of cancer in which abnormal conjunctival cells form a cancerous growth on the surface of the eyeball. This tumor type is more dangerous and invasive than the more common basal cell carcinoma, and it also benefits from early detection, surgical excision, and freezing or cryotherapy.
Some favorable features that consumers should be aware of include the following:
- Polarization: Polarized sunglasses reduce glare from snow, water, highways, and other reflective surfaces. Consequently, wearers see better and squint less. These advantages can be crucial to automobile drivers, airline pilots, heavy-equipment operators, and athletes.
- Impact resistance: Impact-resistant sunglasses resist breakage when dropped or mishandled. Glasses made of a trade-name plastic called CR-39 are lighter than glass and more impact-resistant. Glasses made of high-index plastic are even lighter than CR-39 glasses. In addition, these lenses have a coating that protects them against scratching.
- Photochromicity: Photochromic sunglasses have lenses that brighten or darken in response to the intensity of sunlight. This transitional lens treatment does not in and of itself provide UV protection, a common misconception. Thus, be sure to look for the UV protection rating of photochromic lenses as a separate value.
- Water-sheeting: Lenses with this feature help preserve visual acuity when water (from the rain, the sea, waterfalls, etc.) strikes the lens. With water-sheeting, the water forms in a see-through sheet on the lens rather than individual droplets that block or distort vision.
Many pharmacies (and other vendors) today sell magnifying glasses for viewing tiny objects or fine print. These products are available as handheld magnifying glasses, headset magnifiers, magnifiers that rest on a stand, also known as coil stand magnifiers and magnifiers that can be attached to eyeglasses, sunglasses, or even diving masks. Coil stand magnifiers are particularly well suited to prolonged near tasks because they maintain a consistent focal length, thereby reducing constant refocusing and eyestrain.
The power of these products usually ranges from 2x to 3x (two times the actual size to three times the actual size). Handheld magnifiers generally resemble a big lollipops. Headset magnifiers feature a wrap-around cinch that attaches to the head and a flip-up visor with the magnifiers. The magnifiers can be raised above the eyes or lowered to the eyes as necessary. Magnifiers on a stand allow viewing of a small object while the hands remain free. This feature keeps the focal distance constant, reducing fatigue while freeing the hands for other functions. Magnifiers attached to eyeglasses, sunglasses, or diving masks also enable the viewer to keep his or her hands-free. Some magnifying glasses contain a light to illuminate viewed objects. These products require batteries or an electrical connection.
Inexpensive plastic magnifiers are also available. They may or may not be flexible. These devices may be attractive because of their lightweight, handiness, and of course their low cost. However, they tend to scratch easily and have a short lifespan. It is generally best for anyone who needs a good magnifying glass not to skimp but invest in high-quality products.
Eye doctors specializing in low-vision aids can provide the best advice and the best products for magnification and vision enhancement, particularly for people with eye diseases and compromised vision. These professional products are particularly well-suited for patients with macular degeneration or hereditary visual conditions.
"Visual impairment in adults: Refractive disorders and presbyopia"
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