Autofocus

The autofocus system of the K-70 houses some fresh improvements. The camera gains the option of continuous AF in movie mode, using its on-sensor phase detection, but this is only unlocked when using specific lenses (it appears to be only DC lenses, or PLM lenses, no screw-drive or SDM lenses seem to work). It also promises improved viewfinder AF response, and the algorithm for focusing on moving subjects is claimed to have been improved.

The camera can automatically select an AF point, either from all 11 of its points or from a cross-shaped selection of 5 AF points. Beyond this, you can manually specify an AF point or use an 'Expanded Area' mode where you specify a point and the camera considers the neighboring points, then finally there's a Spot AF mode that locks the camera to the central point.

Telephoto viewfinder AF

Let's start with autofocus through the viewfinder with a distant subject:

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We've shot this test in Expanded Area mode, with the target started on the central point. This means the camera will try to track the subject if it moves within the nine cross-type AF points near the middle of the frame (i.e. the center point plus its eight immediate neighbours). This means the tracking can only venture a small distance from the center of the frame, meaning the photographer also has to work to keep this central region on their subject.

As you can see in the sequence above, tracking performance here was fairly good, though we expect this level of performance for a single subject well-isolated in depth (without any distracting objects at similar distances). There are a couple of missed frames though, indicating that sudden changes in acceleration can stump the camera.

Getting the best results meant experimenting with the camera's AF-Hold setting, which defines whether the camera should pause before refocusing in response to objects passing in front of the subject. Once dialed-in, distractions shouldn't cause the camera to suddenly jump to the foreground, nor should the camera jump to the background and stay there when the subject falls off the AF points, which occurred when we set AF-Hold to off.

It's important to note that this good AF performance was limited to just one lens: the 55-300mm PLM. When the same settings were applied to the kit 18-135mm lens, suddenly the camera had problems keeping up, apparently hindered by the slower autofocus motor. Performance was improved marginally by switching to the 70-200mm F2.8 for a couple rounds, but our best hit rate was with the 55-300mm PLM, which we now feel fairly comfortable saying is one of the fastest, if not the fastest, focusing lenses in the Pentax lineup, and it's nearly silent to boot.

Keeping track of AF Tracking

That isn't the whole story. The AF point indicators are very small and are only illuminated when focus is acquired. This can make it difficult to tell whether the camera is successfully tracking your subject, as you'll just see a brief flash of the AF point the camera confirms is in-focus. Rival systems tend to constantly indicate which AF point is being used (regardless of whether focus has been achieved), showing more clearly that the camera is following the correct subject.

Close-distance subject tracking

Although the K-70 performs well at longer distances, we wouldn't expect to see such good results at close range. For a start, the AF system is solely based on distance information, which puts it at a disadvantage compared with systems that use image information (either from the main imaging sensor or from the metering sensor) for the detailed subject recognition often needed at close range.

Secondly, we only achieved the camera's full performance with the PLM long zoom and no such option exists for testing on the wide end. This meant we were left with screw-drive primes and the slow 18-135 kit zoom, which can't be driven either quickly enough to offer state of the art continuous autofocus performance at close focus distances.

That aside, the improved tracking performance sits on top of good AF-S performance for the nine cross type points. The outer two, which aren't cross type, don't focus as accurately as the inner nine, and can appear to hunt from time to time.

While the Pentax is a class leader in many regards, AF is merely - at best - on par with some DSLRs, worse when used with the majority of slower-to-focus lenses, and behind the best of its mirrorless and DSLR peers.

AF in Live-View

The phase detection points don't change the live view autofocus behavior with many lenses. There's still a contrast-detect hunt, which is very noticeable and loud when performed with one of Pentax's screw-drive pancake primes. Speed and accuracy with most lenses isn't improved over the K-S2. However, mount the DA 55-300 F4.5-6.3 ED PLM WR RE out front, and things change greatly. AF-S acquisition speeds up. Since this is a hybrid system it by nature still does a hunt, but at a much quicker pace than any other Pentax lens we tried.