Sorry if I wasn't clear. I was using a single focus point. The Sigma sometimes just doesn't engage auto-focus on my D810. For this shot I tried various placement of the AF sensor on my son's face, but still nothing. AF did engage when I tilted down to grab the toy and then recomposed. Doesnt happen the majority of the time, but happens enough that I'm now soured on the lens. Doesn't happen with any of my Nikon lenses in similar light/subject contrast. Haven't done a controlled test in varying light and with subjects of varying contrast, or with using LV for AF. I'm not interested in performing QA for Sigma, I just need the lens to work
Could just be a bad copy, but I've seen several posts of people experiencing the same behavior with this lens (and I think the 50mm f/1.4 Art too). I own the latter lens and it happens with it as well. Bought the two too close together to have noticed the issue between purchases. Uncommon enough with both lenses that it took me awhile to notice a pattern. But after some time using both, the frustration factor from missed shots has pushed me over the edge.
The release mode is somewhat beside the point as it isn't an issue of whether the shutter trips or not, but rather whether AF will engage at all for the selected focus point. Similarly it's not an issue of continuous vs single acquisition AF.
I appreciate your experience with the "auto" focus point selection. I've experienced the same thing--often it nails it but at times it misses spectacularly