Autofocus

Words by Rishi Sanyal, Photos by Dan Bracaglia

Introduction

While Sony may stress that the a77 II shouldn't be directly compared against the top-of-the-line cameras offered by its competition, on paper its specification list might suggest that it could take on top competitors. Sony has packed many pro-grade features into the A77 II aimed at pro-grade performance and autofocus. The camera can shoot at up to 12fps with continuous AF, and the 79-point AF module has been designed from the ground up to offer a wide field of dense coverage across the APS-C frame. In particular, the potential of the AF system might almost be seen as a justification for the SLT system - having an image sensor tell a traditional AF module what to focus on arguably combines the best of mirrorless and DSLR form factors. With that in mind, this page will focus largely on how the A77 II stacks up in this regard.

79-point AF module

The a77 II features a 79-point AF system with a large area of coverage across the frame.

The a77 II has a newly designed, dedicated phase-detect module for fast, continuous AF. 79 AF points offer a very large area of focus coverage. 15 of those 79 AF points are cross-type, indicated as green squares. Cross-type AF points ensure more definitive focus in tough lighting situations, including backlit and low-light conditions when there's low subject contrast, so it's a bit of a shame they're limited to the central portion of the image. The red square at the center is for lenses with a maximum apertures of F2.8 or faster, which results in more precise focusing.

What does SLT offer?

SLT technology allows light entering the lens to be simultaneously directed at the main image sensor as well as a dedicated phase detect AF module. This allows both the AF module and the image sensor to 'see' the scene simultaneously. Furthermore, the lack of a mirror that needs to flip out of the way for the shot to be taken means that high frame rates can be achieved.

What's the advantage of having both the AF module and the image sensor 'see' the scene simultaneously? Well, for one, it means you've got an electronic viewfinder (EVF) paired with a traditional phase detect AF system, which isn't possible on any other type of DSLR. Perhaps more importantly, though, the main image sensor provides subject recognition for the identification and tracking of, say, faces, while the phase-detection AF modules provides fast autofocus and tracking due its understanding of subject distance. Having the two systems talk to one another opens up a lot of possibilities - the image sensor can fix on a subject (or you can assign a subject to fixate on), and then tell the traditional AF module what to focus on.

When a face is detected and in focus, a green square will appear around it. The a77 II focuses on and tracks faces remarkably well, a great demonstration of the benefits of combining an image sensor for subject recognition and a traditional phase-detect AF sensor for fast continuous AF.

And that’s where Sony’s SLT system has a unique value-add: it combines the best of both traditional DSLRs and mirrorless cameras, taking the phase detection principles that have been perfected over years and years of iteration, and marrying it with the subject recognition and tracking capabilities of a high resolution image sensor. Today, mirrorless cameras can do the latter quite well, but to make matters more complicated, so can metering sensors in certain DSLRs.

Most Nikon DSLRs can track subjects by using its dedicated RGB metering sensor for subject recognition, and the Canon 1D X and 7D Mark II also do something similar. However, the resolution of these separate metering sensors is rather limited, ranging from 2,016 to 150,000 total pixels. These metering sensors are essentially image sensors, but very low resolution ones. Therefore, one might expect the A77 II to be better at isolating and tracking subjects than Nikon or Canon DSLRs, given the use of its high resolution imaging sensor to understand subjects.

Furthermore, the a77 II's dedicated phase detection AF module always ‘sees’ the scene, since the mirror doesn't flip out of the way when a shot is taken. This allows it to continuously track a subject during continuous shooting without ever ‘blacking out’ - an issue that plagues traditional DSLRs. So on paper, the a77 II has the right hardware to potentially compete - and perhaps supersede - the AF systems of the ‘big guns’ (Nikon D4 and Canon 1Dx, for example).

But does it live up to its hype? Sometimes, yes, and other times, no.

Subject tracking with Lock-On AF

The ability of a camera to recognize faces and/or track subjects using a high resolution image sensor is a unique advantage of cameras operating in ‘Live View’ or EVF modes (e.g. all mirrorless cameras), though the actual implementation and its effectiveness vary wildly from manufacturer to manufacturer, camera to camera. The a77 II, like a mirrorless camera, is constantly scanning the image sensor; therefore, it can track subjects across the frame by analyzing its high resolution sensor feed to stay locked on a subject. What the a77 II brings to the table is traditional phase detection on top of this, which can gather a lot of light per AF point, and are adept at understanding the distance to a subject to immediately focus on it.

Lock-on AF: general performance

But the a77 II doesn't live in a vacuum, and the reality is that its subject tracking, which Sony calls 'Lock-on AF', is often easy to trip up, despite having the theoretical benefit of a high resolution image sensor for subject recognition. You can see this in the video below:

The a77 II can subject track reasonably well, but is easy to trip up with faster moving subjects. In the video above, 'Lock on AF', which attempts to track an initially chosen subject (whatever you first focus on when you half-depress the shutter button) gets confused at the 3 and 10 second marks. Once we re-lock-on to the striped cat, it does fairly well.

While the A77 II should, in theory, function as well as 3D tracking on a hypothetical Nikon D750 with an incredibly high resolution metering sensor (we single out the Nikon here because it has the best subject tracking currently available amongst DSLRs). But it doesn't.

In fact, tracking a subject after initiating focus on an intended subject (using Lock-on AF) was somewhat laggy and inaccurate - the camera eventually caught up & found where the subject had moved to, but with some delay, and some wandering off to other elements in the scene. Sometimes, the AF point never came back to our subject (say, the eye of a face), instead wandering off somewhere else: to the nose, or to some other subject entirely. Lock-on AF also appeared to perform better when the camera was oriented in landscape (vs. portrait) orientation - a rather puzzling result.

Lock-on AF: 'jumpiness'

What was particularly noticeable was that Lock-on AF led to quite a bit of ‘jumpiness’ where the lens would significantly hunt back and forth. This meant there was a significant risk that the lens would hunt to a focus distance well away from the initial subject at the moment of capture– leading to a very out-of-focus image. In the video, we show the lens' focus distance scale as we move the camera left and right, where distance to the subject does not change. Ideally, the focus distance wouldn't change much at all (and it doesn't with the same test on a Nikon camera, for example), and yet it does every now and then, signifying the lens jumping off to significantly different focus distances while the camera attempts to track the initial subject - the eye of our mannequin head.

The subject distance scale tends to jump around a lot, even though we are only tracking the mannequin head across the frame and back, with no depth movement.

Since the camera was quite good at tracking a subject as long as the AF point was stationary (that is, a subject moving toward/away from the camera, but still falling under the selected AF point), we figure this is largely up to the camera’s subject tracking algorithms. Because the 'Lock-on AF' tracking algorithm tends to get confused, the camera might momentarily jump to a different subject entirely, at a different distance, thereby throwing off focus on your initial, proper subject. Eventually, the camera does seem to catch up, but by then you may have already taken your shot, or your subject may have moved - both of which will lead to out of focus images. And indeed, we got a lot of out of focus images using Lock-on AF, despite meticulous technique, variation of AF settings, and proper AF microadjustment for every lens we used with the camera. We think this is down to the subject tracking algorithms getting far too easily confused, and being far too eager to jump off the subject to something else (even if it does eventually return to your subject).

Lock-on AF: inconsistency

To put this in perspective, though, oftentimes the camera will track an initially chosen subject well and focus on it, it’s just that for as many times as it seems to do this right, it also seems to get it wrong. Our perspective is skewed by Nikon’s 3D tracking on its full-frame bodies, which is uncannily consistent at tracking subjects down to eyeball accuracy (meaning it can stick to an eye on a face if that’s what you initiated focus on, granted the face isn't too small in the frame). Compared to some of the other competition, the A77 II may perform admirably - it certainly subject tracks better than a 5D Mark III, which has little understanding of the subject other than the depth it was initially at and has no face detection AF at all, and even slightly better than the 7D Mark II's 'Intelligent Tracking and Recognition' (iTR). It’s just that the a77 II could have been class-leading in this regard, given the advantage of a live image sensor for constant subject analysis, yet isn't.

Why should you care?

We had high hopes for subject tracking in continuous AF with the a77 II, given its combination of an image sensor for subject recognition, and a traditional phase-detect AF sensor for fast autofocus and depth-tracking. Our testing indicated, however, that subject tracking ('Lock-on AF') is so inconsistent and erratic that it often does more harm than good, and you'll get better results by simply leaving it off in continuous AF and focusing the good ol' way - selecting your own AF point, as we've done below in our basketball shot. On our next page, we'll take a closer look at continuous AF in other modes.

Scenes with confusing backgrounds, or fast moving subjects, proved to be too much for the a77 II's lock-on AF. This one was instead shot using AF-C with a flexible spot. After failing for a quarter to get nearly anything usable with lock-on AF, I switched to good old fashion continuous AF with a single point. Shot at ISO 3200, f/4, 1/500 sec.