[sirpilade]

Yes the explanation is diffraction. As light passes through a lens, diffraction acts in similar way as a light through a small pinhole. Incidentally, pinholes and apertures are low pass filters.

Some more info here

Miles V. Klein, Thomas E. Furtak - Optics 2nd ed, Wiley

Joseph W. Goodman - Introduction to Fourier optics, W.H. Freeman