OPTICAL CORRECTION

VERTEX DISTANCE

The power required in a 
contact lens may be quite 
different than that in the 
patient’s spectacles.

One reason the power may 
be different is due to the 
distance the lens is from 
the eye.  For a spectacle 
lens this distance may be 
12 to 15 mm, called the vertex distance.  A contact lens is on the cornea.  For high prescriptions (over 4.00 D.) moving the lens from the spectacle plane to the cornea can make a significant difference.  

For example, a patient who requires a power of -8.00 D. in a spectacle lens at a vertex distance of 12 mm will only require -7.30 D. in a contact lens at the cornea.