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Some clinics do not have 6-metre eye lanes available, and either a half-size chart subtending the same angles at 3 metres (9.8 ft), or a reversed chart projected and viewed by a mirror is used to achieve the correct sized letters. In the most familiar acuity test, a Snellen chart is placed at a standard distance: 6 metres.
Each row of the chart depicts optotypes of a different size. Typically the largest optotypes are in the top row. The optotypes become progressively smaller towards the bottom of the chart. The person removes any glasses or contact lenses, and stands or sits a standardized distance from the chart (e.g., 20 feet for the Snellen chart). [14]
Near visual acuity or near vision is a measure of how clearly a person can see nearby small objects or letters.Visual acuity in general usually refers clarity of distance vision, and is measured using eye charts like Snellen chart, LogMAR chart etc. Near vision is usually measured and recorded using a printed hand-held card containing different sized paragraphs, words, letters or symbols.
Oversized sunglasses, because of their larger frames and lenses, are useful for individuals who are trying to minimize the apparent size or arch of their nose. Oversized sunglasses also offer more protection from sunburn due to the larger areas of skin they cover, although sunblock should still be used.
Pinhole glasses, also known as stenopeic glasses, are eyeglasses with a series of pinhole-sized perforations filling an opaque sheet of plastic in place of each lens. Similar to the workings of a pinhole camera , each perforation allows only a very narrow beam of light to enter the eye which reduces the size of the circle of confusion on the ...
CR-39 is about half the weight of glass with an index of refraction only slightly lower than that of crown glass, and its high Abbe number yields low chromatic aberration, altogether making it an advantageous material for eyeglasses and sunglasses. A wide range of colors can be achieved by dyeing of the surface or the bulk of the material.
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