A light passing through a lens will be focused for a converging lens. However, to look at it carefully, it'll not converge to a single point but to a certain size. The size of it and some fringes surround it is actually a diffraction phenomenon. I don't really understand that the usual diffraction phenomenon we learn from textbook is a light source encountering an aperture or an obstacle. But the lens itself is just letting the light beam focused. I don't really see the idea of aperture or an obstacle come in to the lens that produce the diffraction pattern. Thank you in advance for any explanations.