LENS POWER

                 BVP versus FVP

Another consideration in specifying lens power is 
the difference between "front vertex power" and 
"back vertex power".   Clinically we usually specify 
back vertex power (BVP), but occasionally the 
laboratory will specify front vertex power (FVP).

With a contact lens of significant power (over 6 
diopters) there is often a significant difference 
between the two.  This means if you specify or 
measure the power as BVP and the lab makes the 
lens as FVP you will not get the lens you want.

The BVP is read with the back surface against the 
stop of the lensometer-the focal length is 
measured from the back surface.

The FVP is read with the front surface against the 
stop of the lensometer-the focal length measured 
from the front surface.

When ordering a high powered lens (+ or -) be sure to specify BVP.