According to the Egami Blog, Canon has filed a patent for a 135mm f/2 optical design containing two apodization filters.
From Wikipedia:
Apodization is an optical filtering technique, and its literal translation is "removing the foot". It is the technical term for changing the shape of a mathematical function, an electrical signal, an optical transmission or a mechanical structure. In optics, it is primarily used to remove Airy disks caused by diffraction around an intensity peak, improving the focus.Patent Details (Google Translated)