ASPHERIC LENSES

The rate at which the curve flattens 
away from the center is specified by 
the e-value.  An e-value of 0 means it 
is a spherical surface.  The higher the 
e-value the greater the rate of 
flattening away from the center.

e-values between 0 and 1.0 describe 
         an elliptical surface.
an e-value of 1.0 is a parabola
an e-value greater than 1.0 describes
        hyperbolas

Aspheric contact lenses for 
non-presbyoic patients have e-values 
of about 0.4 to 0.5.  Aspheric contact 
lenses used as “bifocals” for presbyopic patients have e-values of 0.9 to 1.2 in order to give plus power in the periphery of the lens for reading.