I've been reading a lot of lens reviews for prime lenses lately, for 35mm, 50mm, and 85mm lenses. I noticed that Zeiss lenses are made directly in a Canon mount, which has intrigued me. For landscape and astrophotography work, a manual-focus lens with a very wide throw is more useful to me than an auto-focus lens with shorter manual throw. In almost all of the reviews I have read about Zeiss lenses, I have noticed something about Zeiss lenses that bothers me a bit. After focusing on a specific point, taking subsequent shots at increasing F/numbers shows that most Zeiss lenses seem to have a focus shift, often quite pronounced.
Comparing similar Canon lenses with Zeiss lenses, I can't say that I've seen the same effect on Canon in most cases, and if there was some focus shift (i.e. EF 50mm f/1.2), it was not nearly as pronounced on the Canon lens as the Zeiss lens. Is there a reason for this? It seems surprising to me that such a high quality, highly rated brand would have such a consistent problem with their aperture/focus. I can only imagine the problems this could cause, the aperture is only stopped down when the shutter is released. Either I would have to use aperture preview on my camera while manually focusing a Zeiss to make sure it is focused correctly (which is just complicated), or just deal with the focus shift... :(
Is there some specific design decision that leads to focus shift like that when stopping down?