Well I sold my first Sigma lens and decided to buy it again. The first one just seemed problematic on this body. Now the new one I just bought still has back focusing issues but so does my Nikon 50 1.4 and there both deadly at -20. No room for play but its on the money and BTW the Sigma 35 just smokes the Nikon 50mm 1.4 bt that's not the thrust of this thread though but working with both of them today I noticed something that I think is the issue with the sigma 35mm and why some are having issues. Lets start with the Nikon using the shutter button or the back rear focus button once you hit the focus it stops focusing and there is no hunting . I'm on AF-S single point in the center.
Now I think I found why reverse engineering is problematic . On the Sigma using the same shutter or rear focus button once you hit the focus immediately if you continue to hold before firing it has a tendency to continue to focus and it should not do that but it does and this is where you get inconsistent results maybe 3 out of 7 shots get nailed but the rest are off. Now thinking back the same issue was there on the first Sigma. I'm directly chalking this up to either a firmware issue or this reverse engineering of AF is wrong.
Now I did find a solution and you need to be kind of quick and its a two finger operation. Either the rear focus or shutter button in either case if your initial press than at the same time hit the AE/AF lock button immediately without pause it will hold the focus and it does it almost every time. Now its really hard to hit the rear focus button and the AE/AF lock so you really need to use the shutter and AE/AF lock in tandem. So try this just hold down the Shutter button and get focus on something and keep holding it and you'll notice a very slight shift. Why it does not hold is really the issue its like its still hunting but if you start over throw it out of focus than go back to your test image hit it again than immediately hit the lock you will notice your in focus.
One other trick I found with everything still in AF you want to manual focus hit the AE/AF lock when your on that way it does not hunt while holding down the shutter release.
Now love to see some others try this and see if it works. This does give me some thoughts as I remember I had some of the same focus errors with my old Sigma 150 2.8 which if I recall had the same issue. This brings up some interesting problems with AF and Sigmas. Am I seeing something that no one else has noticed or is it maybe my body. But this is pretty hidden if your not watching directly for it. Now I don't want to scream foul either but I find this kind of odd. But the Sigma 35mm 1.4 is maybe one of the sharpest lenses I seen wide open. It's certainly no question the best 35mm lens I have had and that's up aganst the Nikon 35 1.4 g and the Zeiss 35mm F2 so it is up against some serious glass .