by John Walters
Q: My new prescription is very different from old.
It has a +.75 and +.50 Cyl.
While my old had -.75 and -.50.
New Axis 015 and 165
Old was 98 and 64.
Can that be right? I'm afraid to order new glasses with the new prescription because it is so different. A:
The truth is that the new prescription is not very different from the old one, do not worry.
As I explained on our site, the cylinder can be written two ways: + and -. As long as the numbers are the same, the cylinder is the same.
There is a math calculation that decides the spherical diopter, but the cylinder is the same. When you transform from - to +, you add 90 to the axis so from 98 you add 90 and is 8 degrees ( from 180 you start again from 0). So actually you have the -0.75, but the axis changed from 98 to 105 but is transformed mathematically to 15.
There is a formula that is saying: the cylinder can be written two ways, but is the same, only the sign is changed + or -.
For an example I will transform the numbers you gave me:
Old prescription: -0.75 Cyl 98 degrees
New prescription: +0.75 Cyl 8 degrees.
These two are the same.
There is also a spherical part to the prescription that changed, but I don't have it, so we stay only on the cylinder.
You have 15 degrees, so your axis changed from 8 to 15, this means a real change, but not a big one.
Hope this helps.