Autofocus Performance

One of the major selling points of even entry-level SLT models is capture speed. And as we noted on the first performance page of this review, Sony continues that trend with the A99. Of course, capture speed is only half of the equation for action and sports photography. So with its flagship SLT camera, Sony has taken the opportunity to enhance autofocus capability by introducing a dual AF sensor design.

The main 19-point AF sensor (11 of which are cross type) is placed not in the base of the mirrorbox, but in the top, where a conventional DSLR's pentamirror/prism would be. Light reflected from the semi-transparent mirror feeds the AF sensor constantly, even during image capture (still and video). Because of this, the camera allows continuous AF at up to 10 fps (6 fps in full resolution mode). New to the A99, this is augmented by 102 assist points on the main imaging sensor itself. These assist points mean the camera constantly receives distance information about the subject, even when it's between the primary 19 AF points of the main AF sensor.

The A99 has a 19-point AF array with continuous focus possible at up to 10 fps (6 fps in full resolution mode). The focus points are somewhat clustered towards the center of the frame.

The ability to accurately track fast-moving subjects largely determines how many keepers you come away with. To that end, Sony has also introduced an AF depth assist tracking mode, an in-camera focus limiter and simplified activation of its object tracking feature found in previous Alpha series cameras.

Depth Map Assist AF

At the top end of the market Sony has traditionally struggled to match the sophisticated subject tracking AF systems of Canon and Nikon. Decades of catering to the demands of professional sports photographers have helped the big two to refine their focus systems in a way it's hard for Sony to keep up with. However, while they can build on this history of research, conventional DSLRs are able to acquire relatively little information about the subject they're trying to track. They have to rely on information from their dedicated AF sensor and from any color information they can glean from their metering sensors.

In contrast, Sony's SLT design means the A99 is always capturing phase detection data from its AF sensor, and both color and depth information from its newly phase-detection-enabled main sensor. In theory, this gives it a much clearer understanding of the subject it's trying to track and crucially, where it's moving.

The A99 offers a new focus option, AF-D mode, for predictive AF powered by a secondary 102-point phase-detection sensor. You can access AF-D mode through the Quick Navi menu (shown here) or via the customizable silent controller.

With the A99, Sony introduces Depth Map Assist AF technology to offer predictive autofocus. When enabled, Depth Map Assist AF works by collecting contextual information about a subject once the camera has achieved initial focus using one of its 19 AF points. This then helps the camera to predict subject movement between exposures. By using this additional information, the user does not need to specify so much information about the expected behavior of the subject as is necessary when configuring the AF tracking systems on pro-grade Canon and Nikon DSLRs (a process that has become easier in the latest generation, but still requires careful consideration to get the best results).

Surrounding the A99's 19-point AF array is a 102-point AF assist array that provides subject information in order to predict subject movement.

In practice, this dual AF system provides a marked improvement over previous Sonys, particularly when paired with the AF range limit feature we'll describe shortly. But we would hardly classify the A99 as a sports photographer's camera. At indoor events, like collegiate basketball games, the AF system can be a bit slow to lock onto fast moving subjects coming straight towards or away from the camera. In the 6 fps burst mode we often found more precise focus on the second or third frame. And once the camera does achieve sharp focus, the predictive autofocus of the AF assist points cannot reliably keep up with subjects running at full speed. In burst mode sequences shot under these conditions, we rarely found more than two consecutive frames in sharp focus.

No one could reasonably expect the A99's AF performance to be on par with the much more expensive pro-oriented Nikon D4 and Canon EOS-1D X cameras. And while Sony has made obvious strides over earlier Alpha-series models, a direct peer like the Canon EOS 5D Mark III offers a nearly equivalent frame rate, wider AF array coverage and a more robust predictive traking system. The A99 does have the advantage of simplicity of operation. Set the camera to AF-D mode and choose from among the AF area options. That's literally all there is to it. And for the photographer looking to occasionally capture recreational or family sports, the A99 can produce good results, if you set your expectations accordingly.

Specific issues

One caveat to be aware of, however, is that the depth assist mode only works with a selection of available Sony lenses. As of this writing, the only compatible lenses are the Sony 24-70mm F2.8 ZA SSM Carl Zeiss Vario-Sonnar T*, 28-75mm F2.8 SAM, 50mm F1.4, 300mm F2.8 G SSM II, 70-400mm F4-5.6 G SSM and the 500mm F4.0 G.

An even larger concern is the small size of the 19-point AF area. The 102 AF-D assist points cannot be used to acquire focus, so your subject must initially fall within an area of the frame that is smaller than the 39-point array found on the less expensive Nikon D600 (as illustrated below). Sony does provide the option to crop the frame to a narrower field of view via the Smart Teleconverter option. For situations where a smaller 10MP (1.4x crop) image will suffice, this has the decided advantage of allowing the AF array to occupy a significantly larger proportion of the frame.

Overlaid on this image is the 19-point AF array of the SLT-A99. The red rectangle surrounding that array represents the wider coverage offered by the Nikon D600's AF array, which is itself relatively small by enthusiast DSLR standards. As you can see, it's not difficult to imagine situations in which the A99 will struggle to capture focus on subjects moving quickly outside the central area of the frame.

It's also worth noting that the D600 packs 39 (versus 19) AF points into its array, and at 5.5 (versus 6) fps, provides a nearly identical frame rate. The D600 is also a significantly less expensive camera.

One distinct disadvantage to current EVF technology is that when shooting bursts at higher frame rates, you are not seeing a live preview, rather the frames you have just captured. This can make camera panning (to follow a fast moving subject) virtually impossible to do with any accuracy.

Lest we sound overly negative, its worth remembering that unlike the decades old DSLR AF systems, the approach Sony has taken is still in its early stages of development. As such, while it's quite possible that the SLT design will be looked back on as a stop-gap measure, the other way of interpreting this is that it's the only way of offering this combination of capabilities, using current technologies.

AF Range limit

AF Range acts as a virtual, adjustable focus limiter. If you know your subject's distance from you is only going to vary within a certain range, you can tell the camera not to focus outside that distance range - preventing the camera getting distracted by movement in the foreground or background. Pan with a racing car and, with AF range set up properly, the camera shouldn't attempt to refocus on a post that appears part of the way through your pan.

Pressing the 'AF Range' button on the back lets you define a virtual focus limiter. As you fine-tune the near and far limits of this range using the rear and front dial respectively, the display shows which focus points are currently registering an object within the selected range.

On this display the white arrow indicates the lens' current focus position while the yellow crosses and boxes show which AF points feature a subject within the chosen range. Boxes are conventional AF points, crosses are on-sensor AF points.

And, just in case you're not expert at estimating your subject's distance from you, there's the option to preview which objects are within your selected range. As you reduce the focus range, fewer and fewer of the camera's 121 focus points will light up yellow, letting you isolate your subject and prevent the lens hunting outside that range.

In practice, we found that the AF limiter is essential to getting consistent results with telephoto lenses when capturing action and sports. And a useful, intuitive interface makes the feature fast and easy to configure. With the AF Range Limit set appropriately, we avoided massive lens hunt when using focal lengths as long as 300-400mm. We much prefer the convenience and greater range of options provided by having this control in the camera as opposed to using the much more basic lens switch.

Object Tracking

The ability to access two series of AF sensors and the image data at all times gives the A99 an advantage when it comes to subject tracking, meaning it has a better understanding of where the subject is. With the A99, Sony has made the most of this by simplifying and speeding-up the method used for locking onto a subject. In previous recent implementations, you had to press a button to enter focus tracking mode, then press it again to confirm the subject - not really practical with the sorts of moving subject you might with to track. The A99 gives you the option to begin tracking simply by half-pressing the shutter button. And rather than simply selecting the central subject, the camera will give weight to the closest subject.

Sony has featured focus tracking on its cameras for a couple of generations now, but its usefulness has always been undermined by the amount of time taken to engage it.

Now, rather than having to press multiple buttons and risk missing the moment, you can now enable an 'object tracking with shutter option' and acquire a subject lock simply half-pressing the shutter button.