When wearing glasses, the distance from the front of the cornea to the back surface of a lens is called the Vertex Distance. Why is this measurement important?
Prescription lenses in eyeglass frames have different effective powers at different distances, tilts and wrap angles from the eye. That means that the same prescription on two different people, one wearing lenses farther than the other would actually have different powers. Lenses become more plus (less minus) when moved away from the eye (increasing the vertex) and less plus (more minus) when closer. That explains why telling a progressive or bifocal wearer to slide their glasses to the tip of their nose provides a stronger add to read the phone book or a medicine bottle.
TO MEASURE VERTEX DISTANCE
Using a mm ruler, measure from the back of the lens to the front of the cornea
If you can’t see the back of the lens - measure from the front of the lens to the front of the cornea (then subtract lens thickness).