# Eyesight question, or, converting -5.5 into 20/whathaveyou

**URL:** <https://boards.straightdope.com/t/eyesight-question-or-converting-5-5-into-20-whathaveyou/388376>\
**Category:** Factual Questions\
**Created:** [January 17, 2007, 9:10pm UTC](https://boards.straightdope.com/t/eyesight-question-or-converting-5-5-into-20-whathaveyou/388376 "2007-01-17T21:10:24Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Birdmonster](https://avatars.discourse-cdn.com/v4/letter/b/71e660/32.png) [@Birdmonster](https://boards.straightdope.com/u/Birdmonster)\
**Post date:** [January 17, 2007, 9:10pm UTC](https://boards.straightdope.com/t/eyesight-question-or-converting-5-5-into-20-whathaveyou/388376/1 "2007-01-17T21:10:24Z")

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Millions,

So I just went to the eye doctor and got a new prescription. It’s -4.5 on each eye. How can I convert that to 20/20 or, rather, 20/300? I tried googling, but I just don’t know the vocabulary to use.

No longer walking into trees,  
birdmonster

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**Author:** ![bibliophage](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bibliophage/32/7615_2.png) [@bibliophage](https://boards.straightdope.com/u/bibliophage)\
**Post date:** [January 17, 2007, 9:57pm UTC](https://boards.straightdope.com/t/eyesight-question-or-converting-5-5-into-20-whathaveyou/388376/2 "2007-01-17T21:57:40Z")

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To help with the vocabulary, [visual acuity](http://en.wikipedia.org/wiki/Visual_acuity) is what is measured as 20/20, 20/300, etc. The figure -4.5 is the [refractive error](http://en.wikipedia.org/wiki/Refractive_error), measured in [diopters](http://en.wikipedia.org/wiki/Dioptre).

There is some correlation between visual acuity and refractive error, but but my understanding is that they really measure different things so there is no one-to-one conversion. You can find various tables on the Internet that purport to relate the figures, but I think they should be taken with a grain of salt. If you want to know your uncorrected visual acuity, ask your eye doctor or find yourself a [Snellen chart](http://en.wikipedia.org/wiki/Snellen_chart) and estimate it yourself.

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**Author:** ![Birdmonster](https://avatars.discourse-cdn.com/v4/letter/b/71e660/32.png) [@Birdmonster](https://boards.straightdope.com/u/Birdmonster)\
**Post date:** [January 17, 2007, 10:19pm UTC](https://boards.straightdope.com/t/eyesight-question-or-converting-5-5-into-20-whathaveyou/388376/3 "2007-01-17T22:19:34Z")

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**bibliophage** : Thanks. That was really helpful, plus, “diopters” is the second best word I’ve learned today. No one’s taking the gold medal from “foofaraw” though.