Why can't a prescription for glasses be used for contacts also? Isn't the eyesight still the same?

At first, I cynically thought it was just another way for my optometrist to make money -- so he had to charge me for another test. But then I thought about it... and I understand that the contacts need to fit to your eyes and such, so they need to know how bulgy or small your eyes are, etc -- but does the overall eyesight prescription stay the same?

Or is the reason, as I have theorized for the second prescription for contacts, they are testing the size of your eyes and getting measurements?

1 Answer 1


Partly the additional sizing information required for contacts (Diameter and curvature of the cornea are needed for contact lenses but not for glasses) and the power also changes because the physical distance between the eye and the lens is different.


Here is a page explaining the differences between the two prescription types with explanation as to what the different values mean.

  • 1
    Good answer, can you provide reference as mentioned by Narusan?
    – DoctorWhom
    Sep 24, 2018 at 15:46
  • @Narusan no worries, can totally understand that. I answered from my own knowledge but will see if I can find suitable references. The different measurements taken/recorded on contact lens / eyeglass prescriptions are described on Wiki but it doesn't cite anything specific so I'll have a dig and see what I can find and I'll edit them in. Sep 24, 2018 at 16:05
  • 1
    I've added one reference for now - will keep looking for some more authoritative ones. Sep 24, 2018 at 16:13

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