# I know my eyeglasses perscription, but how far am I from 20/20 vision? **URL:** \ **Category:** Factual Questions\ **Created:** [February 7, 2003, 9:57pm UTC](https://boards.straightdope.com/t/i-know-my-eyeglasses-perscription-but-how-far-am-i-from-20-20-vision/153577 "2003-02-07T21:57:52Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![Lamia](https://avatars.discourse-cdn.com/v4/letter/l/8e7dd6/32.png) [@Lamia](https://boards.straightdope.com/u/Lamia)\ **Post date:** [February 7, 2003, 9:57pm UTC](https://boards.straightdope.com/t/i-know-my-eyeglasses-perscription-but-how-far-am-i-from-20-20-vision/153577/1 "2003-02-07T21:57:52Z")
I am _very_ nearsighted. My left perscription lens is 4.75. I also know that my corrected vision in my left eye is somewhere between 20/20 and 20/15, so I correct to slightly better than normal. What I don’t know is how much worse my uncorrected vision is than 20/20. Having 20/20 vision means that you can read at 20 feet the letters that someone with normal vision could read at 20 feet. Having, say, 20/50 vision means that you need to be 20 feet away to read letters that someone with normal vision could read at 50 feet. I can understand that system. I have no idea what my perscription number means, except that the higher it is the worse my vision is. I’m wondering if there’s some formula I could use to convert my perscription into something I can make sense of. I’m imagining something like “20 \* 4.75 = 95, so Lamia’s uncorrected vision in her left eye is 20/95”, although if there’s a real answer I wouldn’t be surprised if it were a little more mathematically complex than that. I attempted to Google this, but since I don’t know enough of the technical terms involved I just came up with a bunch of sites that wanted to sell me glasses or insurance coverage. ---
**Author:** ![zqhltb](https://avatars.discourse-cdn.com/v4/letter/z/f04885/32.png) [@zqhltb](https://boards.straightdope.com/u/zqhltb)\ **Post date:** [February 7, 2003, 10:23pm UTC](https://boards.straightdope.com/t/i-know-my-eyeglasses-perscription-but-how-far-am-i-from-20-20-vision/153577/2 "2003-02-07T22:23:39Z")
IANA optometrist, but I have -4.25 in one eye and -4.75 for the other. Both eyes are 20/400, and from talking to my optometrist, I’ve learned that the “20/400” category is pretty broad, ranging from about -3.5 or -3.75 and up. It is my understanding that after 20/400, they stop using that convention, and refer to it in degrees of blindness. Good to know that I’m not the only one who has to squint at the monitor sometimes 🙂 ---
**Author:** ![Padeye](https://avatars.discourse-cdn.com/v4/letter/p/a9a28c/32.png) [@Padeye](https://boards.straightdope.com/u/Padeye)\ **Post date:** [February 7, 2003, 10:50pm UTC](https://boards.straightdope.com/t/i-know-my-eyeglasses-perscription-but-how-far-am-i-from-20-20-vision/153577/3 "2003-02-07T22:50:41Z")
Those number measure different things though they are related. 20/20, 20/400 is a relative measure of how well you see compared to a person with “normal” vision. 20/20 does not mean perfect vision. -4.25 and -4.75 measure the power of your lenses, a negative correcting nearsightedness. Two people who can focus correctly at 20 feet, with or without lenses, may have slightly different visual acutity. One may be able to read smaller print even though both people are in good focus. ---
**Author:** ![barbitu8](https://avatars.discourse-cdn.com/v4/letter/b/839c29/32.png) [@barbitu8](https://boards.straightdope.com/u/barbitu8)\ **Post date:** [February 7, 2003, 10:51pm UTC](https://boards.straightdope.com/t/i-know-my-eyeglasses-perscription-but-how-far-am-i-from-20-20-vision/153577/4 "2003-02-07T22:51:15Z")
Eye charts only go up to 20/300. It’s meaningless trying to convert the diopters on your _prescription_ (note spelling) to 20/?. 20/4… would, however, be much worse than 20/400. Before my RK surgery in 1991, I was about -7 in each eye, which is actually more than twice as bad as -4… -4.25 or -4.75 is quite nearsighted, but many people are much, much more nearsighted. The prescription is not measured in “degrees of blindness,” but in diopters, and if you search this MB you will find a recent thread explaining what that is. ---
**Author:** ![Ferret\_Herder](https://avatars.discourse-cdn.com/v4/letter/f/e47774/32.png) [@Ferret\_Herder](https://boards.straightdope.com/u/Ferret_Herder)\ **Post date:** [February 7, 2003, 10:51pm UTC](https://boards.straightdope.com/t/i-know-my-eyeglasses-perscription-but-how-far-am-i-from-20-20-vision/153577/5 "2003-02-07T22:51:34Z")
I work as a researcher in an ophthalmology department. I talked to one of the optometrists, and he says that if you’re nearsighted your prescription should be a -4.75 rather than +4.75. He says it’s hard to convert precisely in this fashion, but typically for negative numbers, -1.x is about 20/100, -2.x is 20/200, and so on. So you should be somewhere between 20/400 and 20/500. The term that you’re looking for, for the 20/x designation, is the Snellen equivalent. ---
**Author:** ![barbitu8](https://avatars.discourse-cdn.com/v4/letter/b/839c29/32.png) [@barbitu8](https://boards.straightdope.com/u/barbitu8)\ **Post date:** [February 7, 2003, 10:54pm UTC](https://boards.straightdope.com/t/i-know-my-eyeglasses-perscription-but-how-far-am-i-from-20-20-vision/153577/6 "2003-02-07T22:54:57Z")
> [@](#): > > _Originally posted by barbitu8 \* > \*\* 20/4… would, however, be much worse than 20/400._\* I meant to say that -4… would be much worse than 20/400. ---
**Author:** ![Lamia](https://avatars.discourse-cdn.com/v4/letter/l/8e7dd6/32.png) [@Lamia](https://boards.straightdope.com/u/Lamia)\ **Post date:** [February 7, 2003, 11:44pm UTC](https://boards.straightdope.com/t/i-know-my-eyeglasses-perscription-but-how-far-am-i-from-20-20-vision/153577/7 "2003-02-07T23:44:36Z")
Thanks everyone for the info. I knew I could count on the Dope! And those of you who mentioned that my prescription should be a negative number are right – I didn’t even know enough to know that the “-” was a minus sign and not just a dash, but now that you’ve explained it I see that it is -4.75. ---
**Author:** ![Super\_Gnat](https://avatars.discourse-cdn.com/v4/letter/s/e274bd/32.png) [@Super\_Gnat](https://boards.straightdope.com/u/Super_Gnat)\ **Post date:** [February 7, 2003, 11:58pm UTC](https://boards.straightdope.com/t/i-know-my-eyeglasses-perscription-but-how-far-am-i-from-20-20-vision/153577/8 "2003-02-07T23:58:50Z")
☹ That means that I’m 20/900. ---
**Author:** ![Mr.Blue\_Sky](https://avatars.discourse-cdn.com/v4/letter/m/d26b3c/32.png) [@Mr.Blue\_Sky](https://boards.straightdope.com/u/Mr.Blue_Sky)\ **Post date:** [February 8, 2003, 12:11am UTC](https://boards.straightdope.com/t/i-know-my-eyeglasses-perscription-but-how-far-am-i-from-20-20-vision/153577/9 "2003-02-08T00:11:18Z")
HA! You call yourself nearsighted? My prescription runs -13.75. ---
**Author:** ![Wikkit](https://avatars.discourse-cdn.com/v4/letter/w/c0e974/32.png) [@Wikkit](https://boards.straightdope.com/u/Wikkit)\ **Post date:** [February 8, 2003, 12:51am UTC](https://boards.straightdope.com/t/i-know-my-eyeglasses-perscription-but-how-far-am-i-from-20-20-vision/153577/10 "2003-02-08T00:51:58Z")
[Previous thread on the topic](http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=157366). Mmmm, snellen…