( ( A ( concave lens ) ) ( ( ( is ( thinner ( in ( the middle ) ) ) ) ( than ( it ( is ( near ( its edges ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (JJ concave) (NN lens)) (VP (VBZ is) (ADJP (ADJP (JJR thinner)) (PP (IN in) (NP (DT the) (NN middle)))) (SBAR (IN than) (S (NP (PRP it)) (VP (VBZ is) (PP (IN near) (NP (PRP$ its) (NNS edges))))))) (. .)))
A concave lens is thinner in the middle than it is near its edges.
(ROOT (S (NP (DT A) (JJ concave) (NN lens)) (VP (VBZ is) (NP (NP (JJR thicker)) (PP (IN at) (NP (DT the) (NNS edges)))) (SBAR (IN than) (S (NP (PRP it)) (VP (VBZ is) (PP (IN in) (NP (DT the) (NN middle))))))) (. .)))
A concave lens is thicker at the edges than it is in the middle.