I think a bit of history here might help..... and I am leaving a lot out here, but here goes.
Asahi bought the name Pentax from Zeiss (Jena - actually there were 2 Zeiss companies but that is a different story). Pentax and Zeiss worked together for a brief time, essentially designing the K mount, and the K28/f2 is the same lens as the famous Zeiss 28/f2 Distagon from Contax. Rumor has it that Pentax was the origin of the Zeiss T* lens coating (from Pentax's SMC coating). For what ever reason, the corroboration fell apart, and Zeiss went into a partnership with Yashica (later Kyocera bought Yashica) to build the Contax camera line. This was a professional line of 35mm SLRs available from 1975 to until 2005 when the Contax line was folded, with the last camera model being the Contax N Digital (the first digital SLR).
The vast majority of the Contax lenses available are in the initial C/Y mount (2 versions the AE followed by the MM). The N mount was the last Contax mount and it is incompatible with the C/Y mount. The C/Y mount is very similar to the Pentax K mount that Zeiss helped Pentax develop. After Contax stopped production of cameras, Zeiss started offering their current line of SLR lenses with their Z line. The Z line produces lenses across a number of mounts - ZE (Canon mount), ZF (Nikon mount), ZA (Sony mount), ZK (Pentax K mount). Zeiss a couple of years ago stopped producing the ZK mount since Pentax did not have a full frame body to take full advantage of the lenses.
The Contax Zeiss lenses in the C/Y mount are the older version of the current Zeiss Z lens designs, which new are pretty expensive. For the most part the C/Y lenses are about 1/3 to 1/2 the new price - however their wider angle lenses - the 18 and 21 mm are about equal in price to the new lenses. The C/Y lenses can be remounted to a number of other mounts via the Leitax.com. These are all manual lenses.
We now come to the Contax N mount lenses. What makes these lenses so valuable is that with a reasonably expensive conversion, they remain AutoFocus lenses on Canon bodies.
So you can make a nice tidy profit by selling the lens to the Canon folks, and then turn around and acquire a Contax C/Y lens - 18, 21, 25, 28, 35, 50, 85, 100, 135, 180, 200 prime convert it to a K mount and you too can enjoy a reasonably inexpensive Zeiss lens on the Pentax body. Only the prime lenses are modifiable to another mount, the zoom lenses are still really only useable via adapters on the 4/3 and Canon bodies.
I have converted and shoot with 2 Contax C/Y lenses - the 28/2.8 and 85/2.8 and also have acquired a Zeiss 25/2.8 ZK
Last edited by interested_observer; 04-27-2012 at 05:14 AM.