Trifocal lenses

Trifocals are eyeglasses with lenses that have three regions which correct for distance, intermediate (arm's length), and near vision. John Isaac Hawkins developed the trifocal lens in 1827.[1]

"Trifocal" redirects here. For the tensor, see Trifocal tensor.

Trifocals are mostly used by people with advanced presbyopia who have been prescribed 2 diopters or more of reading addition. The intermediate addition is normally half the reading addition. So, for someone with a distance prescription of −4 diopters and a reading addition of +3, the reading portion of their trifocals would have a net power of −1, and the intermediate segment would be −2.5 diopters.

Trifocal lenses are made in similar styles to bifocals, but with an additional segment for intermediate vision above the reading section. A common style is the 7×28 flat-top or D-shaped segment, 28 mm wide, with a 7 mm high intermediate segment. Larger intermediate segments are available, and are particularly useful for people who spend a lot of time using computers.

Trifocals are becoming rarer as more people choose to wear progressive lenses.

Astigmatism

Astigmatism is a widespread vision condition that can cause people to have blurred vision. It's caused by an imperfection in the cornea or lens of the eyes. Corneal astigmatism is when the cornea isn't evenly curved and smooth. Lenticular astigmatism is when the shape of the lens is distorted. Both conditions can be corrected by using toric contact lenses.[2]

gollark: If *evolution*... well, "attempts" would be anthropomorphizing it... to cross said chasm, all it can do is just throw broken ones at it repeatedly with no understanding, and select for better ones until one actually sticks.
gollark: If I want to cross a chasm with a bridge, or something, I can draw on my limited knowledge of physics and materials science and whatever and put together a somewhat sensible prototype, then make inferences from what happens to it, and get something working out.
gollark: No. We can reason about problems in various ways. So can some animals.
gollark: It doesn't have its own will. It's a giant non-agent mess driven by tons of interacting blind optimization processes.
gollark: Depends. There's not a general answer which isn't vaguely stupid somehow.

See also

References
  1. Stein, Harold A. (2012). The Ophthalmic Assistant: A Text for Allied and Associated Ophthalmic Personnel (9th ed.). Philadelphia: Elsevier Mosby. p. 205. ISBN 978-1-4557-3346-0.
  2. "Contact lens for Astigmatism". Misaki. Retrieved 2019-12-21.


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