I recently had two eye exams done at two different facilities (one ophthalmologist and one optometrist). I received a prescription from each, and I'm curious if they agree with each other.

One prescription says (for each eye):

-4.00 : +0.75 x 180°

and the other prescription says (for each eye):

-3.25 : -0.75 x 90°

I know enough about the numbers to know that the first number is my main vision correction (for nearsightedness) and the second set of numbers is for an astigmatism, but are these two prescriptions equivalent? If they are equivalent, is one form "preferred" over the other by eyeglass labs?


1 Answer 1


One of those prescriptions is written in plus cylinder form, the other in minus cylinder form. This website describes how to convert between the two

  1. Add the sphere and cylinder powers together; this becomes the new sphere power.
  2. Change the sign of the cylinder power, from minus (–) to plus (+), or from plus (+) to minus (–).
  3. Change the axis value by 90°, remembering that the axis must be a number from 1 to 180. According to this, the two prescriptions you got are indeed the same.

Going from the first to the second:

-4.00 : +0.75 x 180°

  1. -4 + 0.75 = -3.25

  2. 0.75 becomes - 0.75

  3. 180 degrees becomes 90 degrees

-3.25 : - 0.75 x 90°

  • Sorry for the first answer : I actually did read the article I linked to here before I answered, but completely misunderstood that part
    – YviDe
    Oct 17, 2015 at 15:25

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