( They ( ( ( use ( a ( negative ( concave lens ) ) ) ) ( as ( an eyepiece ) ) ) . ) )
(ROOT (S (NP (PRP They)) (VP (VBP use) (NP (DT a) (JJ negative) (NN concave) (NN lens)) (PP (IN as) (NP (DT an) (NN eyepiece)))) (. .)))
They use a negative concave lens as an eyepiece.
(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.