Along with Panasonic's GH series, Sony's alpha line of mirrorless cameras has been truly revolutionary for low budget filmmakers around the world. Its compact body, stunning video images, and reasonable price point have allowed many creators to up their projects to a whole new level. But the a7's most intriguing feature has always been its adaptive autofocus, with face detection being its key tool.
It hasn't always been great. Many users complained that the much-hyped ability was often too slow to recognize when their subjects would start in motion. The new a7R III, however, is noticeably on point when it comes to these matters. To illustrate the difference, photographer Dave Dugdale made a short video comparing the autofocus of each which you can watch below.
Dugdale shows how his 2014 a7S loses face detection autofocus when the subject does those typical, fidgety subject gestures that videographers all know and love, i.e. adjusting their glass, itching every part of their face for no reason, turning profile or completely around, or moving to the very edge and out of frame. His new a7R III, however, stays locked onto the subject no matter what small tick they might have.
"The AF speed has increased, the a7Riii has become tenacious. It will not let go of a face," Dugdale explains. "AF has become much smoother, whereas before you would get a shimmering effect." He goes on to highlight how effective the autofocus is even in low-light situations. Much of the camera's ability to keep its subject straight has to do with its mechanic of prioritizing autofocusing on objects in the center of the frame rather than objects closer to the edge.
Source: Dave Dugdale