I believe the correct answer is: material that the lens is
made from.
Lenses are engineered to have
specific focal lengths, besides the curvature of the two sides of a lens. The
focal range, the distance between your optic nerve and the retins in lens, of
the lenses is determined by the material that lenses are made from.
Explanation: clay by nature is not as strong as the others mentioned in the alternative and it can only be worked on when soft and form desired shape when dry because it is malleable making it an unlikely carving tool
