My sister just got her eyeglasses updated. The sheet she got back has this information on it, in almost doctor-style bad handwriting:
OD -275-25X100
OS -250-50X25
Is it possible to convert that to the 20/20 style of eyesight measurement? If so, what would it be?
Thanks.
To the best of my knowledge, all prescriptions are written to bring eyesight as close to 20/20 as possible.
However, here is a website that describes the elements of a prescription, complete with pictures:
http://world.std.com/~dpbsmith/prescription/prescription.html
OD -275-25X100
Right eye: -2.75 dipoters spherical correction -0.25 cylindrical with the axis tilted 100º
OS -250-50X25
Left eye: -2.50 diopters spherical, -0.50 cylindrical axis at 25º
The other thing missing in the prescription is the distance between the center of the eyes.
There is practically no connection between your prescription and the 20/20 thing which is just a rule of thumb indication and useless to know what correction you need.
Actually, you omitted some decimal points, as others noted, and it should read -2.75, -0.25, etc. (OD is right and OS is left.) This is not terribly bad and is approximately 20/200. She’s a good candidate for LASIK, provided that she has a thick enough cornea and does not have dry eyes. 
Although 20/200 and worse would be “legally blind” if uncorrectable, the person still retains a lot of useful vision with 20/200.
As Sailor says you’re comparing two numbers that aren’t measuring the same thing. One is measuring the refractive power of the lenses, the prescription numbers, and the other is a standard for what “normal” eyes can see at 20 feet. Focus and visual acutity are two different things. A person can have the ability to focus perfectly at 20 feet or be corrected to perfect focus at 20 feet but this alone doesn’t mean “perfect” vision if such a thing exists. Some people have exceptional visual acutity that can be significantly better than 20/20.
In China they also multiply diopters by 100 so that they say 275 for 2.75 diopters.
Actually, I didn’t omit anything. I copied it down straight from the sheet, and there are no decimal points. Doesn’t seem to have hindered comprehension.
You’re the only one who has attempted to give a value for that. Could you tell me what that’s based upon?
Thanks for the replies, everyone.
I’m -3 without correction in my OD and I’m also 20/200 OD.
The prescription in the OP is about the same if you add spherical and cylindrical corrections which is what I have done all my life. With such a small cylindrical correction you can just ignore it and get only spherical and you will hardly notice it and the lenses are cheaper.
Actually, the astigmatism OD is -1, and, as an opthalmologist tennis partner explained to me, divide the astigmatism by half and add it to the myopia. OS is plano, but with astigmatism of -3, gives me myopia of -1.5. This is after RK surgery I had in 1991 (actually two on my left and three on my right, but the third did not improve the eyesight).