Comparison

Carl Zeiss Planar T* 50mm f/1.4 (C/Y) vs Pentax SMC Pentax 50mm f/1.2

The Carl Zeiss Planar T* 50mm f/1.4 (C/Y) is a 50mm f/1.4 Contax/Yashica lens made 1975-2005, valued at $120 to $420 used. The Pentax SMC Pentax 50mm f/1.2 is a 50mm f/1.2 Pentax K lens made 1975-1990s, valued at $300 to $800 used.

Specifications side by side

Carl Zeiss Planar T* 50mm f/1.4 (C/Y) and Pentax SMC Pentax 50mm f/1.2 specifications compared
Specification	Carl Zeiss Planar T* 50mm f/1.4 (C/Y)	Pentax SMC Pentax 50mm f/1.2
Focal length	50 mm	50 mm
Maximum aperture	f/1.4	f/1.2
Native mount	Contax/Yashica	Pentax K
Production years	1975-2005	1975-1990s
Optical design	7 elements / 6 groups, Planar Double Gauss, T* coating	7 elements / 6 groups, Double Gauss
Typical used value	$220	$480
Value range	$120 to $420	$300 to $800
Adapts to	Sony E, Canon RF, Nikon Z, L-mount, Fuji X, MFT	Sony E, Fuji X, MFT, Canon RF, Nikon Z, L-mount

What actually differs

The Pentax SMC Pentax 50mm f/1.2 is 0.4 stops faster: f/1.2 against f/1.4.
The Carl Zeiss Planar T* 50mm f/1.4 (C/Y) is cheaper. Typical used value is $220 against $480, a gap of $260 (roughly 2.2x). Ranges: $120 to $420 versus $300 to $800.
Both are 50mm lenses, so framing is identical on the same sensor.
Production of both began around 1975 (1975-2005 and 1975-1990s).
They use different mounts, Contax/Yashica and Pentax K, so each needs its own adapter. Both reach infinity on mirrorless bodies with a plain mechanical adapter.
Both use 7 elements in 6 groups.

Which one to buy

Choose the Carl Zeiss Planar T* 50mm f/1.4 (C/Y) for the $260 lower typical price. Choose the Pentax SMC Pentax 50mm f/1.2 for the 0.4 stops of extra speed. Rendering differs too: read the character notes below, because speed and price are the only advantages the catalog data can settle.

Minimum focus distance, weight, and measured sharpness are not in the catalog, so this comparison does not claim anything about them.

Full reference pages

Mount references: Contax/Yashica and Pentax K