Autofocus Overview and Performance
Sony’s Alpha A9 features a very sophisticated and intelligent focusing system with over 693 phase-detection autofocus points offering 93% frame coverage. AF calculations have been sped up to 60 fps as well, with 25% faster initial acquisition. Most importantly, their approach to continuous autofocus exploits the benefits of an electronic shutter system like never before allowing the A9 to shoot long bursts of 24 Megapixel images at up to 20fps in complete silence, without any vibrations and with no viewfinder blackout.
The key to the A9’s blazing speed is how it uses its electronic shutter. Electronic shutters have long been able to shoot silently at high shutter speeds and without vibration, but what makes Sony’s implementation on the A9 unique is the sheer speed at which its stacked CMOS sensor can retrieve data. This is what allows it to minimize the traditional issues such as rolling-shutter while delivering a live image in the viewfinder with no blackout or lag.
With its incredible 693 phase-detect AF points across 93% of the full sensor area, the A9 offers a vast improvement over the A7 series of cameras. Compare that to the current A7r Mark III which has 399 phase-detect AF points across 45% of its sensor area, and it becomes clear that Sony is saving the best focus system for the A9. While more focus points don’t mean better focus performance, the increased frame coverage does hold a distinct advantage. Unlike Mirrorless cameras, DSLRs employ separate phase-detect AF modules which tend to concentrate their AF points around the center of the frame, leaving a thick border around the center focus cluster where the camera won’t autofocus. Even on cameras such as Nikon’s D5 and Canon’s 1Dx Mark II, the focus modules only cover an area roughly the size of an APS-C sensor. In contrast, the A9’s phase-detect coverage occupies 93% of the frame, allowing it to continue tracking subjects even as they approach the extreme edges or corners of the frame. This works especially well when employing Lock-On AF where the camera can continue tracking a subject as it moves across the frame with incredible confidence. Having such enhanced frame coverage also allows for greater compositional versatility and renders the Focus and Recompose technique so pertinent to composing with DSLR’s almost obsolete. The A9’s focus points spread across almost the whole of the camera sensor which means that you no longer have to use one of the center focus points to attain focus and then recompose because you can just use one of the peripheral focus points found near the corner of the frame. This makes the process of composing with an off-center subject almost seamless and gives the A9 a distinct advantage over its competitors.
Sony quotes the new AF system as working at light levels down to -3EV with an F2 lens. While this compares favorably with the competition, with Canon’s EOS 1Dx Mark II also working down to -3EV and Nikon’s D5 leading at -4EV, I found that the Sony struggled to focus in very dim lighting situations. Part of the problem was using a lens (Sony FE 100-400mm f/4.5-5.6 GM) that has a max aperture of F4.5 at 100mm and F5.6 at 400mm, but I found that my Canon 1Dx with the Canon 200-400mm (even when used with the internal x1.4TC) was consistently better at locking on to subjects in very low light. On the flip side, when shooting in very high contrast situations or trying to focus on high contrast subjects such as the sun, the camera performed admirably and never failed to lock focus.
In terms of AF areas, the A9 gives you the choice of Wide (which chooses focus for you from across the entire array), Zone (which allows you to concentrate the AF on one of nine smaller areas), Center, Expand Flexible Spot (which works like Flexible Spot set to the Small size but with a small expansion of points around it), and Flexible Spot (which lets you manually position a single AF point of three different sizes).
If AF-C is enabled, you can also choose Lock-on AF which tracks a subject based on its shape and color, surrounding it with a shapeshifting frame that changes form and size depending on where the subject is in the frame. Lock-on AF is available with Wide, Zone, Center, Flexible Spot (small, medium or large), or Expand Flexible Spot. To start Lock-on AF, you place the focus point over a subject and then keep the shutter half-pressed for the camera to subsequently track your subject (or what it thinks is your subject) as it moves across the frame.
So, does it work in practice? Amazingly well in fact. Initial autofocus acquisition is very fast, and I had no hesitation engaging the focus system the moment the target was in the viewfinder. With a single subject in the frame against an uncomplicated background, the camera would almost instantaneously lock onto the subject, and rarely was there a single frame that wasn’t in perfect focus. It seems like there is an algorithm in the firmware that is looking for whatever is moving in the frame to target and it does a tremendous job of locking onto any subject movement. This is especially evident when the subject is tiny within the frame. Most cameras, even my 1Dx, tend to struggle to nail the focus in situations where the subject is small and doesn’t dramatically stand out from the background, but the A9 does a great job of latching onto whatever is moving in the frame and stick to it. Take for example the two photos below.
In both cases, the birds are quite small in the context of the whole frame, and neither one particularly stands out from the background. I locked focus on both of them before the frames were taken and the camera was able to perfectly track them as they moved across the frame. Their small size relative to the frame would make it hard for even the best cameras to know exactly what it is that I was trying to track. Furthermore, in cases with small and fast-moving subjects, it is exceedingly difficult to keep your focus point directly on the subject throughout the entirety of the sequence. With Sony’s Lock-on focus, you don’t have to worry if your set focus point is on the subject because the camera does the tracking for you as the subject moves across the frame.
A big test for the A9 was when I used it to photograph the local Black Hawks. Some of the most challenging subjects for our cameras’ autofocus systems are those that move towards the camera at erratic rates of speed. This forces the camera and lens to make rapid and incremental focus adjustments that challenge even the best focusing systems.
The Hawks have an interesting relationship with the local fishermen who throw them leftover fish which the hawks dive to catch with incredible speed and agility. During the first second or two, the hawk moves in a somewhat predictable manner as it slowly glides towards its quarry. The closer it gets to the fish, the quicker it moves, and by the time it dives down to make the catch the hawk is moving at a rapid pace. The whole sequence takes a mere few seconds and it takes great timing and tracking to follow the action consistently.
In these situations, the Sony A9 kept-up with the hawks remarkably well and I rarely found myself missing the shot. Fluid panning movement is essential, and Sony’s relative lightness in weight dramatically aids in this. In contrast, the 1Dx’s weight, even when used with a smaller lens, feels more clunky and less fluid and I found it harder to track with my Canon system. I am happy to report that Lock-On Focus did an admirable job tracking the Black Hawks. It doesn’t mean it was perfect, and I found that Lock-On does tend to be a bit jumpy, but my overall impression was highly positive. Every now and then, the camera would lose focus of the Hawk and latch onto the background, but I was impressed with how well the A9 is able to require focus and do so with incredible speed.
I was also highly impressed with Lock-On focuses ability to track a subject moving across a busy background. There’s a sense of relief when you see the shapeshifting green boxes successfully follow your desired subject as it crosses the frame without ever starting to lock onto the branches behind it.
Of course, not everything is perfect. Issues do arise with Lock-On AF when used with very still subjects (such as a perched bird). It seems that Lock-On seems to latch onto whatever is moving in the frame and with a subject that lacks movement it can get confused and lock onto other objects in the frame. Lock-On AF also struggled in situations with many simultaneously moving subjects moving parallel to the camera (such as a flock of Flamingo) and would often jump around from one bird to another. In such scenarios, it’s best to stick to either Flexible Spot or Zone and tell the camera where it should focus.
One important feature I found missing from the A9 is an ultra-small and precise focus point. The Canon 1Dx has this, and it is an excellent feature to have in circumstances where you need to have a very precise focus on a very small point. This is especially useful when taking photographs of a heavily obscured subject as it allows you to tell the camera to focus on a very small and precise part of the frame. Due to this, I found the 1Dx much more reliable in nailing focus exactly where I wanted when I was working with a heavily obscured subject.
Overall, Sony has come a long way with the focusing system of the A9, and I was blown away by my experience with the camera in this regard. For the most part, it just works and does what it’s supposed to. Lock-On AF was a revelation. With my Canon 1Dx, which is admittedly not the latest model, I find that to get the best results I must be extremely exact in nailing both my technique and my settings. In comparison, the A9 is almost unfairly easy to use, and for the most part, it does a fantastic job locking on and sticking to your subject. The focusing system found on the A9 is both flexible and extremely proficient, and while it would have been nice to have an ultra-precise center focus point, I feel that Sony has really got it right with this camera.
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