There's a major difference between the FD 55mm F1.2 Aspherical and the more conventional spherical version of this lens. Having owned both, and still owning the aspherical version, I can speak from experience. The ordinary, spherical FD 55/1.2 will be soft wide-open because of spherical aberration, coma and other uncorrected lens faults. It begins to sharpen up by F2.0 and by F2.8 performs like any other normal lens at this aperture.
The FD 55/1.2 Aspherical is sharp at F1.2 and only gets better as one stops down. The aspherical element flat out works, eliminating or reducing spherical aberration, coma and a host of other lens faults. Of course, such performance doesn't come cheap. The lens was premium-priced in its day and still costs a lot today. I use mine on my old F-1 and on my Olympus OM-D (with an adaptor).
Erwin Puts, of Leica fame, tested the FD 55/1.2 Aspherical lens several years back and call it one of the finest 50mm - 55mm lenses ever made. Many photographers agree.
Jim B.