謝謝 cation 大的說明..
只是好奇為什麼設計是這樣呢?
這樣顧客賣新品未上機身把玩時不是會覺得怪怪的?
我看說明書好像也沒對此有說明..

這是我找到的一段解釋:
he Contax Lens Mount: Contax G lenses all use the unique G lens mount. This is a breech-lock mount: machined flats on the lens and body flanges position the lens at the exact depth required, while a separate locking ring engages lugs to hold the lens in place. The theoretical advantage of this system compared to the usual bayonet mount is that there is no sliding friction between the lens and body flanges, so mount accuracy is supposed to be better over time. A practical advantage with an autofocus camera is that since the lens flange doesn't rotate with respect to the body flange, there is no need to disengage the autofocus coupler when mounting or dismounting lenses.

The Contax breech-lock mount does not have a separate locking ring; instead, rotating the outer barrel of the lens by its knurled grip ring secures the lens in place. It's similar to the system used on Canon's FD-n lenses, although less complex because fewer mechanical linkages are involved. However, some photographers unaccustomed to this type of breech lock try to handle it like a conventional bayonet mount, and attempt to remove a lens by twisting the entire barrel (rather than just the knurled grip ring.) Although the Contax has a protective steel pin to restrain the lens against being twisted, it's possible to damage a lens or camera body this way. Just follow the instructions and remember to turn the knurled grip ring -- not the entire lens barrel -- and everything will be fine.

Another consequence of this breech-mount construction is that when the lens is not mounted on a camera body, the aperture ring does not turn through its normal range. This does not mean there's anything wrong with the lens -- only that the inner and outer barrels are in different relative positions. Once the lens is mounted on a camera, the aperture ring will work normally.

說明書上有說明這種情況是正常的，請您再仔細看一下