The aperture ring found on the 15/2.8 Distagon ZF.2 (Nikon mount) version means that a mechanical adapter suffices, with aperture control coming via the lens aperture ring.
Regrettably, no such ring exists on Zeiss ZE mount lenses (for Canon EF), which means that an electronic adapter is required (often not available). Hence the ZF.2 variant (Nikon mount) is generally preferable if one is aiming for cross platform usage. ZF.2 can also be used on Canon with an adapter.
The Zeiss ZF.2 lenses offer aperture control via an aperture ring on any camera with a flange focal distance that allows a mount adapter*, such as the Novoflex Nikon to Leica M adapter for use on the Leica M Typ 240.
In this example, I wanted to establish whether the Zeiss ZF.2 15mm f/2.8 Distagon is a viable candidate for cross-brand usage. This makes a lot of sense for speciality lenses in the emerging crossover and mirrorless market: a lens that can do double duty on a DSLR and on other brands offers superior value.
* The Zeiss DSLR lens evaluations do not stop with Nikon or Canon bodies; all such evaluations go into Guide to Zeiss as lens reviews. The platform(s) are only means to the end of documenting the lens performance.