01-25-2018, 05:45 AM
Dave, things like "notorious fickle" or "never been renowned for it's focus accuracy" are something you bring in like I suspected "at this price I'd be careful what to expect". It's not my experience with the brand, otherwise I would not have bought around 10 lenses from their new "global vision" line. However, I don't have any elder model from them. Can't talk about that part of reputation. And honestly, I couldn't care less, as my Sigmas have no lower keeper rate than my Nikkors. Maybe just luck? I don't see myself as a die-hard Sigma fan, I just admit they got a fat bonus of sympathy when they first came out with the 35/1.4 at half the price of the genuine Nikon yet better in each aspect, except some scales and a more rigid lens body of the Nikkor. but that was compensated by the first go it's kind USB-dock. I'm no war photog, my lenses don't need to resist bomb blasts. 
The 18-35 is tricky to focus with not because it's unreliable, but because often there's too much going on in the AF single frame, at the same time f/1.8 has not much of a tolerance for inaccurately chosen focus points (a little bit of movement can make the AF jump to another distance. The AF frames are rather big on a DX camera. It's like using a caliper to measure the diameter of a rope. The caliper is precise, but how's the rope?
How do YOU explain the lens' reliability on a D500 and unreliability at D750? Do you have other lenses to check the "screw" drive with them, or is the Sigma the only one? If your D750 otherwise has a "good AF", does it have it with "screw" drive lenses as well? I'm just asking because with that mechanism I have experience from my Pentax days a decade ago and could not tell if it was good or bad. At the time I had Pentax (ok), Sigma (ok) and Tamron (sometimes ok) lens/es.
I've no idea what really happens in this case. But (prejudice warning again!) in my opinion: New bodies work best with new lenses. Old lenses developed for old bodies in front of new bodies and expecting the AF could do what it never did before (being 3 × more accurate, i.e.) is something I try to avoid. I'm not saying old lenses necessarily are bad - but AF evolved during the last decade quite a bit.
The 18-35 is tricky to focus with not because it's unreliable, but because often there's too much going on in the AF single frame, at the same time f/1.8 has not much of a tolerance for inaccurately chosen focus points (a little bit of movement can make the AF jump to another distance. The AF frames are rather big on a DX camera. It's like using a caliper to measure the diameter of a rope. The caliper is precise, but how's the rope?
How do YOU explain the lens' reliability on a D500 and unreliability at D750? Do you have other lenses to check the "screw" drive with them, or is the Sigma the only one? If your D750 otherwise has a "good AF", does it have it with "screw" drive lenses as well? I'm just asking because with that mechanism I have experience from my Pentax days a decade ago and could not tell if it was good or bad. At the time I had Pentax (ok), Sigma (ok) and Tamron (sometimes ok) lens/es.
Quote:D AF Lenses
Do not use AF lenses with the lens focus mode switch set to M and the camera focus-mode selector set to AF. Failure to observe this precaution could damage the camera or lens. This does not apply to AF-S lenses, which can be used in M mode without setting the camera focus-mode selector to M.
I've no idea what really happens in this case. But (prejudice warning again!) in my opinion: New bodies work best with new lenses. Old lenses developed for old bodies in front of new bodies and expecting the AF could do what it never did before (being 3 × more accurate, i.e.) is something I try to avoid. I'm not saying old lenses necessarily are bad - but AF evolved during the last decade quite a bit.