Autofocus modes

The Autofocus system on the a99 II is both unique and powerful. It uses a similar selection of (many) area modes to most Sony cameras, seen below:

The major change from the a7R II is the loss of the S, M, and L Flexible Spot modes. We still have Wide for automatically picking subjects and prioritizing faces when face detection is on, Zone which behaves like Wide but with a smaller selection area, Center, Flexible Spot and Expand Flexible spot (which surrounds the flexible spot with 'helper' points to help keep focus on a subject). Finally, we have Lock-on AF variants of each of these modes. Lock-on AF allows you to target your subject (using whichever area mode you've chosen) and then have the camera automatically shift the AF point(s) to maintain focus on it even if it moves around the frame. The long list of AF modes can get confusing, with some modes appearing redundant (Center is simply a special case of Flexible Spot), and we wish Sony would give its modes a re-think or at least allow the user to limit the available AF options for selection.

Design

In an SLT design, the semi-transparent mirror eliminates the disadvantage of having to move the mirror out of the way for every shot that plagues DSLRs. When the mirror moves AF is interrupted, and cannot be read again until the mirror has flipped back down. With an SLT, the mirror constantly diverts a fraction of the light to the dedicated phase detection AF module, with the rest of the light hitting the sensor and giving us our image (or allowing the on-sensor AF elements to make their measurements).

If we look at just the module, we already see a strong improvement over the original a99. The AF sensor has grown from 15 points (11 cross-type) to 79 points (15 central cross-type), and has grown in size to offer similar coverage as its rival professional full-frame DSLRs:

A quick look at the a99 system on top of the a99 II system in the rollover above shows how much the total AF area has grown, making the a99 feel quite limited in comparison.

The AF system doesn't end there. All of the 79 AF points, regardless of type, work with on-sensor AF points to make 'hybrid-cross' auto focus points. Outside of this central 79-point area the camera uses the remainder of its 399 on-sensor phase detection points (indicated as small squares on the screengrab above). These points are sensitive to vertical detail (in landscape orientation). Thankfully, unlike the a77 II, the camera won't lock you into F3.5 in 12 fps mode - you can shoot wider if you desire.

Not every lens attached will be able to use this system to its full advantage though. Third party lenses, for example, only focus using the dedicated AF module and cannot make use of any on-sensor AF points. When the camera is restricted like this, the AF isn't very quick or direct, but it is able to focus in very low light levels. That said, at the center point's claimed minimum of -4 EV, we could only get a focus lock with high-contrast subjects, even with lenses using both AF systems. This puts it a couple of stops behind the performance of our benchmark - the Nikon D750 - which is able to focus on even extremely low contrast subjects at -4 EV. Expect solid performance in the real-world, but with low contrast subjects (like faces) in low light, there are better performing DSLRs.

There's another caveat as well: if the camera is stopped down past F3.5, then, in 12 fps mode the camera can only use its on-sensor focus system. At that speed the camera keeps the aperture stopped down constantly during a burst, and the dedicated PDAF module loses its ability to make phase readings past F3.5. If you have the camera set to 'Dedicated PDAF System Only' instead of 'Auto', the camera will revert to manual focus at apertures smaller than F3.5 at these high burst rates, without warning.

One major benefit of this entire system is the ability to use impressive Sony features like Eye AF in a DSLR-style body that doesn't have some of the drawbacks of the company's smaller mirrorless models (such as a lack of control points and short battery life). The other advantage to this system is it is able to offer 12 fps continuous shooting with autofocus. Let's see what 42 megapixels at 12 frames per second looks like:

Continuous AF test: Center

These tests are designed first to show how well the camera copes with approaching subjects (without having to think about following the subject around the frame), then check how well the camera can identify and follow a more unpredictably moving subject, such as a young child running around.

For this first run above, we set the camera to use the center AF point. This should be the fastest and most accurate point, and means the camera doesn't have to think about which point to use. Towards the beginning of the run when the cyclist is further away, the a99 II consistently would have shots that are just barely back focused until Dan covered more of the frame. The camera's hit rate jumps up as Dan approaches closer to the camera, suggesting that perhaps the phase-detection system suffers from being imprecise, which is likely to have a larger impact for distant subjects with little baseline phase separation. As the subject gets nearer, perhaps the module can make a more accurate reading, or simply finally catches up.

Our 'imprecision' hypothesis stems also from our observation of AF being 'jumpy' - even for static subjects, it's easy to see the focus distance scale on the lens jumping back and forth as if the camera is constantly taking a reading and thinking the subject may be out of focus (something we noticed with the similar AF module in the a77 II). In comparison, Sony's own a7R II garners a nearly 100% hit-rate in a similar test, albeit at only 5 fps.

Real-world Performance: Single Point AF-C

A trip to the basketball courts at University of Washington gave us an opportunity to test the a99 II in a real-world sporting situation. While the camera sometimes performed admirably - particularly in 'Wide' mode where you have no control over what the camera focuses on - we experienced a lot of missed shots presumably due to some of the issues we experienced in our bike tests above.

First, the camera often hesitates to initially acquire focus. Despite keeping the center point over our player of interest below and jamming on the AF button (in Flexible Spot, AF-C mode), it takes a number of frames for the camera to catch up and finally acquire focus. And then lose it.

As you see in the last frame above, sometimes the camera inexplicably falters in maintaining focus on a subject. Below, the camera tracked the subject for a few frames, then completely lost focus, despite our keeping the center point over him the entire time. This erratic-ness and 'jumpiness' of focus is simply not something we often expect from a mode as simple as single point AF-C (remember: we're not using any form of subject tracking here). While even a Canon 1D X II sometimes demonstrates this sort of behavior, the best systems, like Nikon's D5 or D500, don't.

Subject Tracking

Somewhat disappointed with single-point AF-C performance, we turned to something even more difficult: testing the camera's ability to follow a subject around the frame, something the camera should theoretically be able to do quite well given its constant access to its image sensor for subject analysis.

There are several options from Sony's AF modes that will track a moving subject. The first is AF Wide, which is essentially an auto mode that picks the nearest/most central subject (or a face if Face Detect is on), and tracks it until a subject more interesting to the camera is chosen (you have no control over specifying your subject). Sony's more targeted subject tracking mode is 'Lock-On AF', which let's you choose the type and location of the initial focus point, giving more control over which subject is selected, after which the camera will (theoretically) track it. The number of Lock-on modes can be confusing, and we suggest simply using Lock-on: Flexible Spot, placing your selected AF point over your subject, then initiating AF to allow the camera to track it around the frame and refocus on it. We'll see how these modes perform below.

You can adjust the camera's responsiveness to changes in depth by adjusting 'AF Tracking Sensitivity' from 1 (Locked on) to 5 (Responsive). With the setting at 1, the camera will slow down its responsiveness and make the system act less jumpy, but will refocus slower if there are sudden changes in subject distance.

Lock-on AF: Bike Weave Test

We tested Lock-on AF (Flexible Spot) with Dan weaving on his bike toward the camera, and for the most part experienced relatively poor performance at 12 fps (the camera would often lose him), so we dropped the burst rate to 8 fps, which seems to improve hit-rates across the board with this camera:

Aside from significant delays in initially acquiring the subject (which can cost you the shot in fast action situations), Lock-On AF did a good job in this scenario tracking Dan around the frame, though it still misses pixel-level precision in some of the shots. While this is all right performance, a single subject against a distant background is not very challenging, and yet still Lock-On AF struggled when Dan weaved out of the hybrid AF area into the on-sensor AF-only region. Occasionally, the camera would suddenly jump to the background and lose Dan for the entirety of the run once he exited the 79 point central area. Changing the 'AF Tracking Sensitivity' from 3 (Standard) to 1 (Locked on) helped eliminate the sudden focusing error, but the outside points still aren't as quick and precise as the hybrid AF area (odd, given how fast the on-sensor points are on the a7R II).

Lock-on AF: Candid Portraits

Subject tracking can be extremely useful for candid portraiture and event photography, where you specify your subject by placing your selected AF point over it, initiate (continuous) focus, and then have your camera track the subject around the frame and maintain focus on it. Unfortunately, Sony's Lock-on AF continues to be too erratic, jumpy and imprecise to rely on for this manner of shooting. Worse, when the camera loses your subject and you wish to re-initiate, it's (1) often hard to even see the grey AF point to place it over your subject, and (2) there's a lag after you let go of the AF button during which the camera still continues to track your original subject (or whatever it's jumped off to).

And while Eye AF is a formidable option for candid portraiture, it continues to be erratic, jumping to any subject it can find instead of sticking to the one you initiated on. This sort of 'shotgun' approach to focus can be useful, but ultimately requires the photographer to give up creative control if he/she is unable to specify which subject to focus on.

Lock-on AF for Candid Portraiture: I initiated Lock-on AF on my subject's face, yet with only minimal subject movement and recomposition, Lock-on AF got confused and migrated to my subject's shoulder, back-focusing this image significantly (the chosen focus point is shown in red). In general, Lock-on AF is too imprecise for tack sharp focus, especially in complex scenes with multiple targets where a system like Nikon's 3D Tracking would have easily succeeded. Photo: Rishi Sanyal

What's it all mean?

It seems then that the a99 II works a lot like the rest of Sony's cameras: the more control you give the camera for choosing focus, the higher your hit-rate. We were often disappointed with single point AF-C performance, with numerous shots out-of-focus during bursts. Somewhat ironically, AF Wide, where you give the camera complete control over what to focus on, turned out to be one of the fastest focusing modes on the camera. The downside? The subject in focus might not be the one you want. Using Lock-On AF with Flexible Spot to specify and track your subject is often too slow to initiate (or re-initiate), and can be very erratic, often jumping off to entirely unrelated subjects and drastically different depths. It simply doesn't work nearly as well as the market leaders.

Now while the a99 II doesn't quite produce 12 perfectly sharp Raw files every second, the camera and its system offers a combination of speed, resolution, and precision that can't be found in other high-resolution bodies. Compared to its little brother, the a7R II, the a99 II has it beat in both control and speed, thanks to that joystick and higher burst rates. Unfortunately, though, not necessarily always in terms of focus.

In our use, we found a couple other issues with this system. One, is it gets held back by some of the slower screw-drive lenses that struggle with great changes in focus depth, while the SSM lenses work quite well.* Another problem is that it's difficult to see your selected AF point, even as you move them, especially in low light or when you're pointing your camera at low contrast subjects. Having the focus point change color or illuminate while its being moved would easily alleviate this issue. Such issues, combined with all the F3.5 caveats and generally so-so focus performance makes us question the utility of SLT over all-out mirrorless or all-out DSLR.


* With screw-drive lenses, on-sensor PDAF points will not even attempt to refocus if there are drastic changes in depth of the subject that's under your AF point(s).