It's actually really simple, it's distance expressed as a fraction.

My vision is 3/60. That means that I need to be 3m away from something to see the same amount of detail that someone with 20/20 vision would see at 60m.

Your variety of acuity might be a result of different methods to measure your vision, different circumstances (lighting, angle, contrast, etc.) or both. I'm photophobic, so my acuity goes down if the room in which my eye exam takes place is bright.

OK so there's a few different types but the fraction version comes from Snellen charts - they're the ones with a single big letter on top, then 2 letters, 3 letters etc. And usually caps out at 8 or 9 letters

**You can probably skip this next bit if you don't care about the history and a bit of the science behind eye charts**

Anyway Snellen decided to try and define a "standard" eyesight and he did that using the capital letter E because its kind of just 5 horizontal lines - the top bar, the empty but, middle bar, empty bit then the bottom bar and acuity (or resolution if its easier to think of it like that, not entirely correct but we'll get there) is basically the ability to tell apart two points or lines so it's a good candidate really.

Now obviously the closer you are to something, the more of your visual field it takes up, it's all about the angle made in that triangle from the top of the object, the bottom of the object and your eye. So something like dots per inch doesn't really mean much when it comes to eyes and instead we think of angles.

We know a circle has 360° in it but we can split those degrees up even further, unfortunately we came up with this well before we really cared about any kind of metrification so decided to cut a degree up, like it was an hour into 60 parts and call those minutes and to split those minutes of arc into 60 parts and call those seconds of arc. Because circles and clocks are basically the same thing or something apparently... Fortunately for Snellen though when he did his experiments it turned out that the average human who didn't need glasses or have any kind of eye disease etc could see 2 lines when they were separated by *about* 1 minute of arc. So this is where his chart came in, he took that E and made it so each line would be 1 minute of arc with a 1 minute of arc gap between them so is 5 minutes of arc total angular size. That's what a person "should" be able to see

**End of boring history/science bit**

Now we know what people "should" be able to see it's easy to work backwards and say "OK if someone can see something *this* big at X distance then because it's the same angular size, they'll be able to see something *that* big at Y distance" and the standard distance was set at 6m for reasons that don't really matter too much.

So if at 6m you can read the things Snellen thinks you should be able to at 6m (something 5 arc minutes tall) we'd say you have 6/6 vision. If you're at 6m and can see things that you should be able to at 12m (so would be 5 arc minutes tall if you were twice as far away) then we'd say you had 6/12 vision etc. Etc.

Space in hospitals is incredibly valuable so we don't always get to have a 6m room so vision charts were made for rooms based on being 3m away or even 1m away or near vision charts that can be 50 or even 33cm still based on that 5 minutes of arc number. So if you're at 3m reading something 5 min/arc tall that'd be 3/3 vision or if you at 3m can see something that you should be able to at 12m we'd call that 3/12 vision.

So 3/3 vision is effectively the same as 6/6 vision, 3/6 is the same as 6/12 etc. Finally the fractions we use for Snellen are usually 6/x or 20/x, again this is based on that 6m room standard but why 6 and 20? Because 6m is very close to 20 feet :) if it's 6/x you're probably in a country that likes measuring in meters but people have heard of 20/20 a lot more so we often talk about that probably largely thanks to American tv shows and it sounding cooler.

TL;DR - 20/20 doesn't mean you got 20 letters right out of 20, it means you can see at 20ft what you should be able to at 20ft. Some places prefer meters so they'd say 6/6 but it's the ratio that's important, not really what the numbers are

The 20 in 20/200 for example refers to 20 feet. Australia doesn't use feet, so the nearest equivalent, 6 metres, is used instead. So the rough equivalent of 20/20 vision is 6/6.