Does anyone know how this works?
Here's an excerpt explaining how (eye) focus tracking works
full article here;
"Given its access to image data off the image sensor at high speeds, the a7R II is capable of understanding subjects (via pattern recognition), which allows it to stick to an initial subject quickly and accurately. This ensures the camera continues to know what to focus on as the framing changes and as subjects move around within the scene. On-sensor phase detect AF points then ensure the camera actually focuses on what it knows to focus on, and does so accurately since the phase measurements are made by the imaging sensor itself and, therefore, do not suffer from the AF inaccuracies separate dedicated PDAF modules in DSLRs suffer from. That's right, no microadjustment needed, folks.
In fact, it's this accurate phase-detect AF that allows the a7R II to focus the Sigma 50mm F1.4 lens with far more accuracy than a DSLR. We have to microadjust F1.4 primes all the time on DSLRs to get accurate focus out of them, and even then, peripheral AF points may continue to be off (as they may be miscalibrated relative to the center AF point). But when we slapped the Sigma Art on the a7R II, pretty much every single shot was in focus.
You'll note that none of the shots in this video show any sort of systemic front or back focus. Put simply, no calibration is needed for accurate shots at F1.4 with the a7R II, because the system is accurate to begin with, as phase measurements are made on-sensor and do not need any correction factors for optical artifacts like residual spherical aberration. This is potentially game-changing: focus at F1.4 without microadjustment? Yes, please."
