Autofocus

The RX100 IV features the latest iteration of Sony's autofocus system, which we're told benefits from the increased information resulting from faster readout of the (now stacked) image sensor. Essentially, the camera's AF system is able to assess the scene more quickly than its predecessor could. This increased information about the scene and subject is used to further improve focus tracking, which enables the camera to perform some advanced autofocus tricks, like eye tracking in continuous shooting. Sony also claims it's re-worked the AF algorithms so the camera doesn't need to move the lens so much before deciding which direction to drive it in. The improved algorithm also tries to estimate where the correct focus distance is with greater precision, so the focus elements can be driven quickly near to the correct focus point, before slowing down for focus acquisition and exposure. A lot of this isn't easy to objectively test, but having used every iteration of the RX100 series, we can confidently say that autofocus is decidedly more determined and quick, even in lower light, lower contrast situations that have, in the past, posed challenges to these cameras.

AF options

Sony's newest AF system offers a broad range of options that are well worth trying to understand, before you shoot.

There are five different focus area modes (Wide, Center, Flexible Spot, Expand Flexible Spot and Lock-on AF). Within this, the Flexible Spot mode, where you manually position the AF point, offers three AF point sizes. Meanwhile, Lock-on AF, which can only be used in continuous AF mode, offers all the other autofocus area modes as sub-options, so that you can decide the most appropriate means of initially selecting the object you want it to track.

On top of this, there's Face Detection mode, which is selected separately. This will indicate any faces that the camera has detected in the scene but the camera will only focus on these faces if your chosen focus area mode overlaps with the face (so an off-center face won't be prioritized over the subject in the middle of the frame, if you're in Center AF area mode, for instance). There's also an option to teach the camera specific faces, which it will then prioritize over unknown faces.

The RX100 IV offers a four main AF area modes, then Lock-On (tracking) versions of those same modes.

On top of these are the Face Detection and Recognition options, an Eye AF mode and, presumably for video use, a separate Center Lock-On AF function.

It's worth learning how these interact with one another.

Finally, an additional option is Eye AF, now available for the first time on a Sony camera in continuous AF mode. You'll need to assign Eye AF to one of the camera's customizable buttons. In AF-C mode, pressing this button will find and follow the nearest eye the camera can find in the scene. In AF-S mode, it will highlight an eye for around 1 second as the eye is focused on. Eye AF does not require Face Detection to be engaged to work.

Just to add a further level of complexity to proceedings, the camera still offers Center Lock-on AF as a separate function (not to be confused with the Lock-on AF Center AF area mode, with which it's incompatible). So far is we can tell, this is a vestigial feature that dates back to before Sony got the Lock-on AF area mode (triggered with a half-press of the shutter button) worked out. The Center Lock-on AF function is locked onto a subject by pressing the central button on the back of the camera and presumably remains in the camera because it's the only tracking AF mode available while in movie mode.

Continuous AF

In the past we haven't always tested contrast-detect AF systems for their continuous AF performance, typically because they didn't tend to perform well. Things are changing, and the RX100 IV's AF was so good in real-world use that we put it through some of our continuous AF tests.

Single point AF-C

Below, we've shot a moderate-distance subject using the central focus point and AF-C. Note that at these working distances, the camera offers fairly extensive depth-of-field. At the 70mm equivalent end of its zoom, its F2.8 maximum aperture is equivalent to F7.6. To make it easier to see when the camera has shifted focus, we've included a 100% crop both of the subject and of a background item that should become increasingly out-of-focus as the rider approaches. As you can see, the camera performs surprisingly well, considering it's a CDAF-only system. In frame 10, the camera suddenly jumps to focusing behind the rider before catching up again, but generally speaking, this is very impressive performance.

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Subject tracking

In this next test we've used Lock-on center AF area mode (Not the Center Lock-On AF function). The camera was set to follow the cyclist by placing the AF point over the biker when initiating AF (which tells the camera that the subject under the AF point is what should be focused on). The frames before this sequence had the rider approaching the camera directly: hence the AF point starting at the center of the first image we've included.

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Indicated AF points are approximate

As with the rest of our experiences with the Lock-on AF tracking, it's fairly inconsistent. When it gets it right, it's brilliant, but often the system doesn't seem very successful at identifying the subject it's meant to be tracking. Hence, while it can get it right, there will also be numerous occasions where it gets it totally wrong and takes great interest in a totally different subject. You'll see this as the green box - that dynamically outlines your subject - keeps changing, growing and moving to follow your subject but, often, seemingly with a mind of its own. It doesn't exactly inspire the confidence that, well, Sony's own Eye AF tracking does in continuous focus (see below). Even in the situations where the camera has tracked the subject, it can lag behind subjects moving at even moderate speeds (as appears to have happened here). As such, we think it's less that the camera wasn't able to drive the lens to keep up with the moving subject, and more that Lock-on AF's ability to track an object accurately and quickly is somewhat iffy.

We'd like to see Sony continue to work on the accuracy and reliability of Lock-on AF to match the performance of more class-leading implementations (Nikon's 3D tracking, Panasonic's tracking AF, amongst others). Face and Eye detection AF are much more successful, perhaps because they have a clearer understanding of what they're meant to be tracking.

Focus priority

It's also worth noting that the camera does not offer any choice of Focus Priority or Release Priority, instead striking a balance between the two, dictated by Sony's engineers. Contrast detection autofocus systems, which have to constantly move their lenses to confirm focus in AF-C mode, tend not to perform well in Release Priority mode, so it's not too surprising that Sony doesn't offer it as an option. However, we wonder if it might be worth allowing a mode that prioritizes focus even more. In this test and some of the subsequent tests below, you'll note the AF system sometimes plays catch-up. And while overall performance is still very admirable, we wonder if a mode that emphasizes focus over shooting rate might lead to an even higher hit-rate particularly when subject tracking - which places an even higher burden on scene analysis and focus tracking - is involved.

Eye AF

Here's a demonstration of how well Eye-AF is able to keep track of the eye in the scene, even when the camera is being fairly rapidly moved around (equivalent to the subject moving around and the photographer trying to follow them). As you can see, the camera sometimes loses track and drops back to the underlying AF mode that had been chosen (Wide area AF with Face Detection, in this case). It also sometimes jumps from one eye to the other but it does a really good job of making sure it gets at least one of the eyes in focus.

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Unlike Face Detection, Eye AF over-rides your selected AF area mode while the button is held down, being quite happy to select an eye from anywhere in the scene, even if face detection is turned off. This is quite nice, as it makes it easy to quickly force the camera out of whatever focus mode it's in if you suddenly find yourself taking a portrait. Intelligently, if you have specified a particular focus point in the scene, the camera will focus on the eye nearest this point, giving you a nice, fast, easy way to prioritize any face on the fly: place the face over your selected AF point, hold down the Eye AF button, recompose, shoot. If confronted by a series of faces, Eye AF will prioritize 'registered' faces over unrecognized ones. So you'll want to register the faces of people you typically shoot, though if you don't, again, you can prioritize them on the fly by initiating Eye AF with your selected AF point near them.

Eye AF does come at the cost of of one of your customizable buttons (and you'll need to choose one that you can comfortably hold down and still reach the shutter button), but has the advantage that you don't need to stop and change AF area mode: you can just press and hold the chosen button when you're trying to shoot a portrait, then release to return to however you were shooting before. We're really impressed with this implementation.

Note Eye AF does not function in video.

Magnification in Playback

It's a small thing but we're delighted to see that hitting 'magnify' in playback mode now automatically zooms in on the focus point that was used to shoot the image, rather than just magnifying the center of the image. This significantly speeds up the process of checking focus when reviewing the shots you've just taken.