Convex lens are thicker at the middle. Rays of light that pass through the lens are brought closer together (they converge).
A convex lens is a converging lens.When parallel rays of light pass through a convex lens the refracted rays converge at one point called the principal focus.
The distance between the principal focus and the centre of the lens is called the focal length.
Specification of optical glass convex lens
+0,-0.1mm (+0,-0.15mm for diameter>75mm)
Focal length tolerance
+/-1% (+/-5% for 400 series)
Center thickness tolerance
3 arc min