This lens lives in the shadow of the younger sibling, the 50/0.95. The Canon 50/1.4 is sharper, and has higher contrast. The 50/1.2 is native LTM mount, the 50/0.95 is Canon 7 breech-lock, must be adapted for use on the Leica. Prices on the 50/0.95 have skyrocketed 15x over what they sold for 15 years ago.
The Canon 50/1.2 can be found (with patience) for $400~$500. I have both lenses: could never see a reason to convert the 50/0.95 to M-Mount. The cost of the conversion exceeds what I paid for this lens in Dec 2019. The lens came with a beautiful Canon Vt Deluxe as a rear cap.
The 1956 Canon 50mm F1.2 is a double-Gauss 7 element in 5 group "Super-Speed" lens, uses a then-new high-index of refraction/ low-dispersion glass that made it possible to offer such speed using only 7 elements. The Nikkor 5cm F1.1 uses 9 elements in 6 groups. The Zunow 5cm F1.1 is a Sonnar formula, also used 9 elements.
This lens is susceptible to haze and etching from the lubricants used attacking the surface behind the aperture. I suspect that much of the bad-rep is due to haze. When looking for one, be able to inspect and return. Some light haze will lower contrast, of the lens is cheap enough- use a hood.
This one is my third in 15 years, and is the first one with perfect glass. This one is from 1957, as is the Vt Deluxe that it came with- both most likely bought new together.
Start off with numbers for vignetting, focus at infinity.

I took this lens and a perfect-glass Canon 50/1.4 to the Marine Museum for an informal comparison. Mostly shooting the 50/1.2 wide-open and at F1.4.
At F1.2-

And at F1.4.

The Canon 50/1.2 can be found (with patience) for $400~$500. I have both lenses: could never see a reason to convert the 50/0.95 to M-Mount. The cost of the conversion exceeds what I paid for this lens in Dec 2019. The lens came with a beautiful Canon Vt Deluxe as a rear cap.
The 1956 Canon 50mm F1.2 is a double-Gauss 7 element in 5 group "Super-Speed" lens, uses a then-new high-index of refraction/ low-dispersion glass that made it possible to offer such speed using only 7 elements. The Nikkor 5cm F1.1 uses 9 elements in 6 groups. The Zunow 5cm F1.1 is a Sonnar formula, also used 9 elements.
This lens is susceptible to haze and etching from the lubricants used attacking the surface behind the aperture. I suspect that much of the bad-rep is due to haze. When looking for one, be able to inspect and return. Some light haze will lower contrast, of the lens is cheap enough- use a hood.
This one is my third in 15 years, and is the first one with perfect glass. This one is from 1957, as is the Vt Deluxe that it came with- both most likely bought new together.
Start off with numbers for vignetting, focus at infinity.

I took this lens and a perfect-glass Canon 50/1.4 to the Marine Museum for an informal comparison. Mostly shooting the 50/1.2 wide-open and at F1.4.
At F1.2-

And at F1.4.
