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

What actually differs

• Both open to f/1.4, so neither has a speed advantage.
• The Pentax SMC Pentax-M 50mm f/1.4 is cheaper. Typical used value is $110 against $220, a gap of $110 (roughly 2x). Ranges: $60 to $210 versus $120 to $420.
• Both are 50mm lenses, so framing is identical on the same sensor.
• The Carl Zeiss Planar T* 50mm f/1.4 (C/Y) is the older design; production began around 1975 against 1977 for the Pentax SMC Pentax-M 50mm f/1.4 (1975-2005 versus 1977-1984).
• 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

On the catalog data the Pentax SMC Pentax-M 50mm f/1.4 wins on the $110 lower typical price, and the Carl Zeiss Planar T* 50mm f/1.4 (C/Y) gives up nothing measurable in return except its own rendering, described below. Condition and variant matter more than the spec sheet at these prices.

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