Camera fine focus adjustment.

[Fine_Art]

How accurate is fine focus adjustment over a range of distances?

I recently got the D600 in response to unacceptable infinity focus on my Sigma 20 1.8 EX DG prime. The lens is good from macro to medium distances.
I picked up the 85 1.8G which is spot on focus.
I picked up the Sigma 35 1.4 USM (the new one on DxO has as the top wide angle lens). This lens is badly back focused. With a tape measure on the floor focused at the 10" about 5ft from the camera the focus is at 20" much closer to the camera. I adjusted it with the fine focus adjustment to proper focus. I took it out again yesterday for some shots with decent results. It seems to be on at close distance. Farther away It is harder to tell. Sometimes the lens is brutally sharp, other times it seems just OK. I really have misgivings about the lens. If it was an OEM lens I would expect the fine focus adjustment to match the rate of focus travel over the whole range. With a third party lens I am not sure. Any experience you have with this type of thing would be greatly appreciated.

[Ken Bennett]

I assume you're autofocusing the lens? My experience with using the focus adjustment on a Canon 35/1.4 is similar. Using AF, sometimes the lens is brutally sharp, sometimes it's a little off. I suspect it's just the nature of AF, there's a little bit of slop at each point in the system that usually cancels itself out, but sometimes is cumulative and leads to unsharp photos.

If I have to shoot at f/1.4 I tend to refocus a LOT and shoot a lot of photos. (By refocus I mean defocus the lens manually then hit the thumb button again to focus. Single Shot focus is more reliable, as is the center point focus on a 5D2.)