what is the difference between a base curve of 8.4 and 8.6?
The base curve (BC) of a contact lens is a measurement of the concavity of the inner surface - the curve that sits on the cornea. ... A BC of 8.4 mm of radius is more curved, and therefore a tighter fit, than 8.8 mm base curve. People who have steeper corneas require tighter fitting contacts.