Nearsightedness - what the #%& is 20/40, 20/80 etc?

I am nearsighted, and I always saw it measured with a scale that uses negative numbers for nearsightedness (I have -4.00 and -4.25 for left and right eye respectively) and positive numbers for farsightedness. What is this scale's relation to this 20/something scale Americans use?

which describe the curvature of the corrective lens.

the 20/100 measure basically means you need to stand 20 feet away from an object to see as clearly as a "normal-visioned person" see it when standing 100 feet away.

http://www.lpf.com/source/rk/20something.html

Holy fscking crap, 20/400! I'm Mr. Magoo!

...Whenever they ask me to read the lowest line I can read I tell them "well, I know it is an E but I can't see it." Once, in the army, I had to go on regular sick call to get a referral to see the eye doctor. SOP was to give me the eye test. The technician told me to stand on the line and read the chart on the wall. He already had my glasses in his hand and there were many things posted on the wall so I could not determine which one was the chart. I asked hem "what chart" and he responded in an irritated voice to knock it off. I guess that amatuer comedians all think they just thought that one up when getting their eyes checked. After telling me to knock it off he said "Oh, I guess you can't see it." He was looking at my lens curvature and realized I was not joking. So my point, Mr. Magoo, is that it is all relative. Would you like to trade?

has "perfect" eyesight. The "20" refers to distance from a standard eye chart in feet (a little over six metres). If one has "20/40" vision, that means that one needs to be six metres from the examination chart to read what someone with "perfect" vision could read at a distance of twelve metres.

Hope that clears it up for you...

...some table that relates the two?

there isn't a direct relation (my eyesight tests as something like 20/100, plus or minus 20, and I'm at -4.5 diopter, currently).

WWII fighter pilot, first to break sound barrier, had 20/10 sight in both eyes. Claims he still has it now.