what does it mean left eye: +2.75 ( -1.75) on 170°?

Answer 1

It tells how strong your prescription is and your astigmatism.
Also, known as diopters.

An eye glass prescription has 3 numbers.
Example : -4.75 -1.75 45
The first number is the correction required for near or farsighted vision. If it has a minus symbol (-) then it's to correct nearsightedness. In the example above it means myopic four and three quarters diopter. If it was a zero, the person requires no correction for near or farsighted vision. The second number is for astigmatism. The third number is the angle of astigmatism, in this case 1.75 diopters of astigmatism at 45 degrees. So the first number shows the correction required for either near or farsighted vision and the second and third number relate to astigmatism. The higher the numbers the greater correction required.
http://www.kcsupport.org/Common_questions.htm


http://en.wikipedia.org/wiki/Dioptre
http://en.wikipedia.org/wiki/Eyeglasses_prescription