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Key Takeaways
- Autofocus is adjustable and can be effective, but isn't as powerful or dependable as Sony's stills photography AF
- 10-bit S-Log3 footage gives plenty of dynamic range, but not 15 useable stops
- Raw output can be encoded as ProRes RAW by Atomos recorders but the footage requires more work for processing and the tools aren't necessarily available to yield a major benefit over capturing 10-bit Log footage
Autofocus
A lot of pro and high-end videographers don't use autofocus, primarily because it hasn't always been very reliable and because there are well-established techniques of working without it. But that needn't be the case forever. Every project will have its own standards of 'good-enough,' ranging from 'ah, I'm sure no-one will notice' all the way up to 'perfect,' and as autofocus gets more reliable, it'll start to exceed that 'good-enough' threshold for more applications.
We've found the a7S III's AF system to be pretty effective, in our shooting. It's not quite as smart as the stills AF system, but gives you a good degree of control over how fast the focus is driven and how long the camera pauses before attempting to refocus.
The a7S III's video AF system is pretty effective, but not quite as smart as the stills AF system
In order to access the camera's tracking, you need to tap the rear screen (it won't start tracking the subject under the AF point, so you can't preemptively set your AF position). This will then track your subject and will use face or eye detection if you've got that mode engaged and will continue to track your subject even if they aren't facing the camera.
This feels less dependable than the stills mode, and we've found it can occasionally start tracking the wrong thing after a while, but in our experience it's usually pretty reliable and worth the risk of shaken footage that comes from the need to tap.
We'd recommend this way of working if you're trying to get the camera to remain focused on people. There's also the option just to let the camera focus on faces it's found within the scene, which has the advantage that you don't need to tap the screen, but because you haven't specified the person as your subject, the camera may focus on something else if they look away.
Dynamic range
In terms of dynamic range, Sony says the a7S III can deliver up to 15 stops of dynamic range. But, while it's true that the S-Log3 curve is designed to encode around 15 stops of information, it's not necessarily the case that all of this is usable.
Here we've shot a brightness wedge with 13 stops in 1/3EV increments. As you can see, the darkest tones near the left-hand edge are extremely noisy (which would be the case even if the camera produced no noise at all). The rollover below lets you see the footage ungraded, then with Sony's S-Log3 -> REC709 LUT applied, which pushes the shadow tones to a typical final-output level. We've then lifted the shadows in an exaggerated manner, to see whether there it's possible to distinguish between the darkest steps.
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Ungraded |
LUT Applied |
LUT + Shadows lifted |
As you can see there's a lot of noise in those darkest tones, even at base ISO, but there does genuinely appear to be some distinction between steps still. So, depending on your noise tolerance, it's fair to say there's a little more than 13 stops of DR available.
Raw video output
The a7S III is capable of outputting a Raw video stream of up to 60p over HDMI (the stream comes from the full sensor, wheres the camera has to crop-in slightly to deliver 120p). Sony says the stream is 16-bit but it's not clear whether this comes from 16 or 14-bit sensor readout. Either way, the only current option to capture it is in the 12-bit ProRes RAW format using an Atomos external recorder.
We haven't been able to get a clear answer on how that 16-bit signal is converted to ProRes RAW, and the lack of detail from Apple about the format makes it hard to predict what, if any, impact this conversion has. The camera lets you choose whether the output is treated as S-Log3/SGamut3 or S-Log3/CineGamut3, but we believe this only changes the metadata that tells the editing software (an NLE such as Final Cut Pro) how to initially render the file.
We'll be looking in more detail at Raw video and how it relates to Raw in stills in a separate article, but one similarity that's immediately apparent is how much processing the camera is doing on its compressed footage. Not only is the Raw footage presented as 4.2K resolution (4264 x 2408) rather than being downsized to UHD (3840 x 2160), it's also had none of the fine sharpening, noise reduction or in-camera processing that the compressed footage has had applied to it. As such, the direct-from-recorder footage appears a touch soft and rather noisier than you might expect. This lack of processing lets you make decisions about these aspects in post, rather than in-camera, but the extra work needs to be anticipated if you're planning to shoot this way.
For now, don't expect easy access to the greater flexibility you get by shooting Raw instead of JPEGs as a stills photographer
Sadly, Sony does not appear to communicate sufficient metadata in its stream (or the software doesn't understand it), to gain access to all of Final Cut Pro's Raw video tools. The 'ISO*' pull-down for adjusting brightness and the dedicated Raw white balance options are not available. This immediately makes it a little more awkward to access the full flexibility of the Raw video and means you risk ending up with footage that's akin to out-of-camera Log footage, but with more initial work that needs to be done.
We hope that Sony, Atomos or Apple is able to find a way to provide access to these tools (and more, as Raw video support matures), to make it easier to gain the full benefit of this way of shooting. For now, though, don't expect easy access to all the greater flexibility you get by shooting Raw instead of JPEGs, as a stills photographer.
*This pull-down isn't truly adjusting ISO, any more than the 'Exposure' slider in Raw processing software is retroactively changing the shutter speed and aperture you shot with, but then again the ISO standard doesn't cover Raw, so don't take any of it too literally.
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