Autofocus and Tracking

By Dan Bracaglia and Rishi Sanyal

Autofocus Improvements

When I first saw the A7 II, during a sit down in our Seattle offices with Sony reps, we were told that the camera offers "30% better AF." That's a difficult number to verify, even in scientific testing, but I will say, I did notice an improvement in focus accuracy, and less of a tendency for the camera to "hunt," when compared to the original a7. Initial AF acquisition certainly seems faster on the a7 II vs the a7.

Keep in in mind, both the a7 and a7 II have the same number of phase detect points, 119 in total. That's 119 more then the a7S or the a7R, which use contrast detect only. If you're coming from shooting with either the "S" or "R" you will certainly notice a vastly more robust AF experience.

We were also told by Sony that subject tracking had been improved 1.5x. We found subject tracking with the original a7 to be often be untrustworthy. But with the a7 II, you can track with varying degrees of success. More on that on the following below.

Lowlight is one area the a7 II still has a very difficult time acquiring focus. Thankfully, switching the camera to manual focus mode, via the button on the back of the camera, is ridiculously easy. Sony's method for manual focus is painless, and actually kind of fun to use. It simply zooms in the frame when manual focus is switched on, but not until you start turning the focus ring. As soon as you stop turning the ring, it zooms back out.

Subject Tracking

This is an example of subject tracking on the a7 II working well. I was tracking the lead singer (shirtless guy), and waited until he stepped into a portion of the stage with some brighter light to fire off a few frames. All of them were sharp, despite the singer's wild movements and less than ideal lighting conditions. Shot at 1/250 sec, f/1.4 ISO 16,000. Note: This image was processed through Adobe Lightroom 5.7.1.

Subject tracking, or 'Lock-on AF' as Sony calls it, functions with some success, depending on several factors. Lock-on AF allows you to select an initial subject to track simply by placing it under the selected AF point and depressing the shutter button halfway. As long as you keep your finger on the shutter button, the camera will attempt to stick to and focus on whatever was underneath the AF point when you initiated focus.

Subject tracking does work noticeably better than tracking with the original a7, but it does not perform as well as the a6000's subject tracking. Factors like the size of the subject in the frame and the camera's ability to differentiate that subject from the background make a huge difference. The biggest problem we noted was that instead of faithfully tracking exactly what was underneath the AF point when focus was initiated, the camera tries to intelligently pick a subject near that AF point. You'll see this as a green box outlining what the camera thinks is your subject, and this green box will move, grow, and shrink to try and remain on your subject. But sometimes this green box outlining your subject just has a mind of its own. It almost tries to be too intelligent. When it works, it works; when it doesn't, the green box may have totally wandered off to a different subject. You can see this in our first sample below; at 0:19, we initiate focus on the black and white cat, and the green box has a difficult time sticking to it, and eventually wanders off to the yellow horse at 0:31.

Using lock-on AF at f/2.2. The green box indicates what the camera thinks you want to focus on and track. Note how it transforms into a flurry of phase-detect AF points at 0:05; once this happens, the camera seems to only track subjects within the central portion of the frame containing phase-detect AF points (indicated by brackets). The green box also often has trouble identifying and sticking to your subject. Though we initiate AF on the black and white cat at 0:19, the green box has trouble sticking to this subject, eventually wandering off to the yellow horse at 0:31.

Further complicating things is the fact that this green box is sometimes replaced by a flurry of small phase-detect AF points that try to stick to your subject. When these kick in, if your subject leaves the central portion of the image (the phase-detect area, indicated by the brackets), the camera will no longer track your subject. You can see this at around 0:05 in the video above. That said, when the subject re-enters the central portion of the frame, the camera usually does a good job of picking it back up again.

It's worth mentioning that if you set the camera's AF area to 'wide' (the default AF area, where the camera automatically selects which AF points to use), the a7 II only uses the central portion of the frame for focus. Presumably this is because the camera prioritizes the use of phase-detect AF points. If face-detection is on, though, the camera will focus on faces no matter where they appear in the frame.

And face-detection works quite well on the a7 II. The camera will tend to automatically find the largest face, track it across the frame, and focus on it. We found this to work even with fast primes, focusing fairly reliably at F1.4 with Sony's 35mm prime (see photo below). That's no small feat, as any DSLR user might tell you, having experienced the inaccuracies and limited face-detection capabilities of many DSLRs employing traditional phase-detect systems.

Here's an example of the a7 II nailing focus, using face detection, at f/1.4. ISO 1600, 1/125 sec at f/1.4. Photo: Rishi Sanyal

Depth Tracking

For subject tracking to work though, the camera has to actually focus on the subject, and that involves depth-tracking of the subject. Here, the a7 II performs quite well as long as the subject remains somewhat centrally located - where the phase-detect AF points reside (indicated by the brackets in the frame). It's almost as good as the a6000/5100 in this regard. Have a look below at how the camera, for the most part, smoothly tracks the central subject as we move the camera toward, and away from, the subject.

Using lock-on AF at f/1.4. Note how the camera tracks smoothly while we move away and toward the subject. The camera's decision to use a flurry of AF points vs the green box is frankly confusing, but as long as you stay within the phase detect area, depth tracking seems to work fairly well.

It's also important to note that phase detection does not work at apertures smaller than f/8, beyond which the camera reverts to the slower contrast detect AF. The reason being that unlike a DSLR, where the lens remains wide open during focusing until right before the shutter fires, the a7 II attempts to acquire focus through the lens at or near whatever aperture you have set. And apertures of f/9 or smaller don't provide enough of an opening in the lens for the phase detect system to properly work.

Simply put, it's questionable engineering, and we wonder if Sony is shooting themselves in the foot by having their cameras operate this way. It essentially means that continuous AF slows down considerably (due to hunting) at smaller apertures, which is unfortunate if you're tracking a subject and require depth-of-field (or simply considerable room for focus error). Another side effect of focusing at or near the selected aperture is that AF often slows down in low light if you've selected a smaller shooting aperture - the smaller apertures deprives the AF sensors of light. And it's only when it gets really, really dark that the camera decides to open up the aperture fully to acquire AF.

While we surmise that Sony has implemented an algorithm to shoot at or near the selected aperture in order to avoid focus shift, we feel they may have gone a bit too far. We can't imagine that focus shift is a huge issue for every lens ever attached to the a7 cameras, especially when stopped down at very small apertures, where depth-of-field is likely to mask focus shift errors.

Below is a video showing depth tracking at f/9. Notice how the a7 II hunts, unlike in the f/1.4 video above.

Using lock-on AF at aperture of f/9. If your aperture is set to f/9 or smaller, the camera reverts to the slower contrast detect AF. This is obvious in the hunting we see in the video above.