Analysis: Sony a7R II and RX100 IV autofocus systems
Sony recently shook things up with the announcement of the 4K-capable 42.4MP, backside-illuminated full-frame a7R II mirrorless camera, alongside stacked 1"-type sensors in the updates to the popular RX100 and RX10 lines. While stills and video quality are sure to break new ground with these updates, perhaps even more interesting are the autofocus improvements.
The a7R II offers compelling improvements that go above and beyond updating AF in the a7 lineup. In fact, the improvements potentially yield an AF system that may offer benefits over high-end DSLRs, as I alluded to in my recent opinion piece. For example, take a look at the 399-point - you read that right - phase-detect AF coverage across the frame (green squares) of the a7R II compared to Canon and Nikon full-frame peers:
Sony a7R II
Canon 5D III
That'll be quite a boon for off-center compositions, and the dense and wide spread of the AF points means the camera will be able to track your subject despite dramatic movement throughout the frame. Speaking of tracking, the high-speed readout of the image sensor also means fast and accurate tracking of your subject no matter where it moves to within the frame. In fact, the system can be so accurate as to find and track not just a face, but the eyes within a face. High speed readout means it can now do this continuously. And on-sensor phase-detect means the camera doesn't suffer from the inaccuracies dedicated phase-detect modules in DSLRs are prone to.
Oh, and it can even do some of this with A-mount Sony glass, and Canon lenses as well, via adapters.*
Similarly, the RX100 IV and RX10 II get upgrades to the speed of sensor readout, and this means better AF, including the ability to focus on and track eyes in continuous AF like the a7R II. In our brief time with the a7R II and RX100 IV, we put the cameras to work at continuous AF, and here are some of our initial experiences, in the form of videos capturing the cameras in action.
Sony a7R II: Continuous AF with Canon EF mount Sigma 50mm F1.4 Art
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 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.
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.
Furthermore, as addressed in my opinion piece, there are still the issues of low-light AF and focus from extreme defocus, areas where traditionally on-sensor phase-detect AF suffers. We'll be curious to see if the a7R II shows improvement in these areas. So you know, we fully expect it will, as we've seen continued improvement in these areas from generation to generation.
Sony a7R II: Eye AF
Here's where things really get interesting. The video above - shot through the electronic viewfinder using an iPhone - shows what the Sony a7R II is capable of with a native FE mount lens attached (FE 24-70 F4 OSS). That's continuous eye tracking, where the camera automatically finds the nearest eye, and continuously focuses on it, despite rather drastic and fast movement along the X, Y, and Z (distance) axes. No hunting, no jumping off to some other subject, just fast and effective AF on our subject's eye.***
For reference, here's the best implementation of subject tracking (where we mostly specify the eye of the mannequin) in a DSLR we've seen to date, with the Nikon D810. It's actually quite good, and well ahead any other DSLR from any other brand. In fact, the D810 and Sigma 35mm F1.4 Art combo was accurate enough to keep focus on the eye of my fiancée - which I'd specified as my target by initiating focus on it - while I moved around freely to recompose to get the lighting and flare as I wanted it. As good as Nikon's tracking system is though, a system working off high-resolution data from a multi-megapixel image sensor, with 399-point dense coverage, really has the potential for even more pinpoint accuracy.
Imagine the potential for fast and accurate focus with shallow depth-of-field F1.4 primes. Contemplate the fact that this eye AF will work with the new Zeiss Batis 25/2 and 85/1.8 lenses. Not only that, think about the implications for video. We haven't had a chance to check if eye AF is available during video on the a7R II yet (it isn't on the RX100 IV), but we can't imagine why this wouldn't be possible with enough processing power. Regardless, subject tracking using Lock-on AF remains available, so as long as you initially specify your subject, on-sensor PDAF should do a good job of keeping your subject in focus without hunting in video.
Sony RX100 IV: Eye AF
Now that you've seen what the a7R II is capable of with regards to eye AF, we wouldn't blame you if you wanted something similar in every camera. Luckily, Sony offers a similar, rather compelling eye AF feature in continuous AF on the RX100 IV. And it's really, really good, especially when you consider the RX100 IV has a contrast-detect only AF system. In fact, it's the reason I'm planning on upgrading my RX100 III.
Eye AF in continuous focus is new to the RX100 IV and RX10 II, and we have a feeling that what enabled it is the faster readout of the image sensor over previous models. Not only does this bring high frame rate and 4K video to these little cameras, but it also gives the focus algorithms more temporal data to work with. And looking at the speed which eyes are tracked, it certainly seems the camera is speedily taking advantage of this extra data.
Of course the proof is in the proverbial pudding: the camera has to actually focus on the eye in addition to tracking it. And here the RX100 IV does not disappoint: every single shot is in focus on the eye, with the lens set wide open to F1.8, as you can see from the magnified views.****
* What finally enabled the use of on-sensor phase-detect information to drive adapted DSLR lenses? Our best guess is the increased speed of sensor readout. If you have any thoughts or guesses, please comment below.
** Oddly enough, though, we find 'Lock-on AF' to be more unpredictable than subject tracking in 'wide' AF area mode, where the small green squares (indicating PDAF points) follow subjects more predictably than the green square of 'Lock-on AF' that appears to have a mind of its own. This is especially why we'd like to see 'wide' area mode have an option to specify the starting point as a way of telling the camera what it is we wish to focus on, a la Nikon's 3D tracking. So that users can avoid the unpredictable 'Lock-on AF' altogether.
*** We've certainly seen implementations of eye AF in other cameras: on many Panasonic and Olympus mirrorless offerings, for example. However, we've rarely, if ever, seen it combined with phase-detect AF for this uncannily fast and accurate AF.
**** 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.