If you are going to use it on a Contax II/III you have no problem. It will focus down to the closest setting of the rangefinder coupling. On cameras like the Nikon Rf's the depth of field takes care of any small disrepancy between the Contax and Nikon mount. If the lens has been converted to a screw or M-mount, check it out first. A lot of the Biogons were converted to screwmount and some couple to the Leica (and thus to ZM and Bessa rangefinders) but many were made in an uncouipled mount. It is not impossible to use without coupling as the depth of field is substantial.
To my knowledge, Zeiss never made an official screwmount version so most of these conversions were made by outside machinists and technicians. Some were good and some really fouled up the lensmount!
If you are using a M mount camera, my suggestion is to spring for the new ZM 21/4,5. It is actually an improvement over the old version (and that is no mean feat!). This lens is now my benchmark for 21mm focal length optics!
