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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]
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 ...
The international glass code is based on U.S. military standard MIL-G-174, and is a six-digit number specifying the glass according to its refractive index n d at the Fraunhofer d- (or D 3-) line, 589.3 nm, and its Abbe number V d also taken at that line.
Two contacts encode the lens's minimum aperture— f /16, f /22, f /32 or f /45; although no Pentax K-mount lens has ever had an f /16 minimum aperture, OEM lenses often have. The other three contacts encode the lens's maximum aperture; their meaning is dependent on the minimum aperture indicated by the lens.
A bifocal lens with areas of differing magnification Bifocals with separate lenses. In this case, the Swedish ethnologist Jan-Öjvind Swahn [ sv ] . Bifocals are eyeglasses with two distinct optical powers correcting vision at both long and short distances.
A 1951 USAF resolution test chart is a microscopic optical resolution test device originally defined by the U.S. Air Force MIL-STD-150A standard of 1951. The design provides numerous small target shapes exhibiting a stepped assortment of precise spatial frequency specimens.