OPTICAL CORRECTION

             LACRIMAL OR TEAR LENS

Another reason that a rigid contact lens power may be different than the spectacle lens power is the tear layer under the contact lens.  If the back curve of the lens does not match the corneal curvature, the tear layer will contribute power.  This tear layer is called the tear lens or lacrimal lens.

If the contact lens is flatter 
(has a longer radius of curvature) 
than the cornea, the tear lens will 
add minus power.

If the contact lens is steeper 
(has a shorter radius of curvature) 
than the cornea, the tear lens will 
add plus power.  

The tear lens will also correct 
astigmatism equal to the corneal 
toricity (corneal astigmatism).