I should have written can
be a minefield. One little word makes a difference.
I'm not trying to imply that AF fine-tune is not a useful feature to have. Of course it is. There are some lenses on some bodies that are just a little bit out and a simple AF fine-tune adjustment results in a worthwhile improvement without complication.
Before the feature of AF fine-tune was provided on DSLRs, I would simply return a lens if it didn't autofocus accurately. Perhaps my frustration in this regard at the moment is due to my not having returned a recently bought lens as soon as I discovered it wasn't focussing properly on my D7100. I thought I'd fixed the problem with a fine-tune adjustment of +10, using a fairly close target, but later discovered that this fine prime lens, the Sigma Art 35/1.4, was producing abysmal results at infinity, on the D7100.
It required a fine-tune adjustment of the maximum of +20 for distant subjects. The difference between the +10 adjustment and the +20 adjustment at infinity is very obvious, particularly between the apertures of F1.4 to F4.
Now, the whole purpose of buying a lens like the Sigma Art is because it's ultra sharp and can produce a shallow DoF at F1.4. I can see no point in using a compromise AF fine-tune setting of +15, unless one is prepared to use F8.
When I use the lens for close-up work, I'll have to remember to check that the AF fine-tune is set on +10. When I use the lens for landscape, I'll have to check that it's set to +20, or use a tripod and LiveView. However, if I don't want to bother fussing around making changes to such adjustments according to the circumstances, I could just leave the setting on +15 and use F8 all the time.
Another complication I've noticed is the inherent inaccuracy of the autofocussing system regardless of AF fine-tune considerations. I use the AF-ON button and a single focussing square so that focussing is independent of metering. By pressing again the focussing button each time one takes a shot of the same scene, from the same position, focussing on the same spot, one gets an indication of the focussing inaccuracy of the system. It's as though one has altered the fine-tune setting for each shot. However, such differences are only apparent at wide apertures. At F8 there's no problem.