LENS DESIGN

         AXIAL EDGE LIFT (A.E.L.)

When fitting rigid lenses we are 
concerned about the amount of 
clearance at the edge of the lens.  
One way of quantifying changes in 
this clearance is by what is called 
axial edge lift.  Axial edge lift is 
the distance from the extension of 
the base curve of the lens up to the 
back edge of the lens as measured 
parallel to the axis of the lens.

For average rigid lenses this is in the range of 0.08 mm to 0.20 mm.  As the peripheral curves are made flatter or wider the AEL value increases.

AEL is also referred to as the z-value.

The edge clearance of the lens on the eye does not equal the AEL because the cornea gradually flattens in the periphery.  However, a given change in AEL will give the same change in edge lift on the eye.