The 21mm f/2.8 Carl Zeiss Planar T* is an autofocus rangefinder lens using a Planar design's tremendous potential while adding modern updates. Because the lens is wide, an external viewfinder can be attached to the camera to get the proper view of your frame.
The Contax 21mm Planar G is a 2nd generation Contax G system lens originally introduced alongside the G2 body, it's also useable on G1 bodies that have been modified to adjust the finder correctly. The 21mm focal length offers a moderately wide field of view that many photographers choose as a primary walk-around lens often opting for it's more encompassing field of view over Contax's other Planar G design, the normal 45mm f/2 G. The 35mm G produces excellent sharpness so many photographers reason that they can crop the image some if needed.
G1/G2 Lens Compatibility
The original Contax G1 is not compatible with the 21mm, 35mm and 35-70mm lenses. Contax provided a firmware upgrade for many of these cameras. Such upgraded G1s may be identified by a green-colored sticker in the film chamber - they are often called 'green label' G1's.