The material I've created was this (2xGlossy/falloff):

but this was the result when I applied it on the lens:

Why are there those concentric circles? How can I solve it? I tried also with 16000 samples, but the result doesn't change.
The centre of the circle isn't a light point (I use a daylight environment) and this effect remains also if I delete the two panels in front of the lens. Could someone help me?
My Pc has two Titan GeForce Black Edition

Thank you,
Andrea