We adapted a Sigma 50mm F1.4 Art lens in Canon EF mount to the Sony a7R II via a Metabones Smart Adapter III. Rather than simply test the acquisition speed of AF-S, we did something far more challenging and interesting: test the AF capability of the a7R II with Canon mount glass in continuous AF mode, with subject tracking enabled to track the subject not just along the Z (depth) axis, but also across the frame, along the X and Y axes.
Given its access to image data off the image sensor at high speeds, the a7R II is capable of sticking to an initial subject quickly and accurately, ensuring that the camera knows *what* to focus on as it moves 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* b/c 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.
In fact, it's this accurate phase-detect AF that allows the a7R II to potentially focus the Sigma 50mm F1.4 lens with far more accuracy than a DSLR. You'll note that none of the shots in this video show any sort of systemic front or back focus. No microadjustment is needed for accurate shots at F1.4, because the system is simply 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.
At the end of the video you'll see that, unfortunately, all the AF modes are not available with 3rd party glass on the a7R II. At least not with our Metabones Smart Adapter III (there is a Mark IV, though) and Sigma 50mm Art of Canon 24-70 F2.8L II lenses. Therefore, it's difficult (if not impossible) to specify what it is you want the camera to focus on, which makes this continuous AF mode of potentially limited value. We hope to see some of these modes enabled by Sony for 3rd party glass in future updates, if not at the very least a way of specifying to the camera what the initial subject is.
In fact, the reason the nose is sometimes focused on instead of the eye in these shots is that, in order to get subject tracking where the camera automatically picks the appropriate AF point(s) to stay on our subject, we had to leave the camera in 'auto area' mode. And so the camera chose the nearest target. This is not an issue for Sony lenses, where you can use 'Lock-on AF' to specify the subject, and even use eye AF. Still, we'd really like to see this ability with 3rd party lenses, since the camera is clearly capable of focusing with pinpoint accuracy on features such as the eye, even with the Canon mount lens used here.
Of course, do keep in mind that you absolutely can specify the AF point with 3rd party lenses if you don't want or need subject tracking across the frame. Flexible spot, in S, M, and L sizes, is available, and you can position the focus point anywhere in the frame, and still get continuous AF (that is: depth-tracking). But part of the charm here is the camera's subject-tracking ability, and the only way to get it with 3rd party lenses is in 'wide' area mode, which does not allow you to specify a starting point to 'define' your subject.
And in case you missed it: 'magnify' in playback now magnifies the AF point used at capture, rather than simply magnifying the center of the frame. No more scrolling over to the point of interest in playback to check focus; you now have one-click (albeit somewhat laggy) magnification of what was focused on. This extends to all three cameras: a7R II, RX100 IV, and RX10 II.
