Panasonic Lumix DMC-GH4 / Sony Alpha 7S Comparative Review
Image Quality: Sony Alpha 7S
Sony's CMOS sensors have shown themselves to be capable of capturing extremely high levels of dynamic range at low ISOs - they add so little read noise (electronic noise) that each generation has allowed you to pull more and more information out of the shadow regions of files. This leads us to expect a lot of the a7S.
This shot was exposed so that the brightest tones were just about to clip to white (in order to capture the maximum amount of light without overexposure).
The a7S doesn't offer quite as much dynamic range as the a7 and a7R at base ISO, with an apparent trade-off having been made to offer the excellent high-ISO performance. That said, there's still an awful lot of clean, usable information that can be pulled out of the shadow regions of the file. Here we've shown the extreme dark patches of the image raised by around 4EV and you can see both color and texture with only the slightest hint of noise beginning to encroach.
This isn't a scene that requires as much dynamic range as the a7S is capturing but shows that the camera can cope not only with scenes such as this but also even higher-contrast situations.
|This image is a +3.5EV push of an image exposed to protect highlight detail.
Close examination of the image reveals false color and artifacts, particularly around high-contrast edges, probably stemming from Raw compression. It's rare to see but means the Sony doesn't have as much processing latitude as you'd otherwise expect. For comparison, you can download the GH4's image of the same scene.
In common with recent Sony models, the a7S compresses its Raw files. In very rare situations this compression can limit the flexibility of the Raw file in post-processing. The compression is designed to retain all the visually-meaningful information from the image as-shot. The gaps in the retained information can very occasionally become apparent with the kinds of extreme processing pushes used for expose-to-the-right dynamic range optimization and will be particularly apparent at high-contrast edges.
Low light image quality
In addition to its resolution being better-suited to producing 4K video, the a7S's relatively modest pixel count also helps it to offer excellent pixel-level performance in low light (smaller pixels may produce more noise individually but this will tend to average - unless they're very small pixels at the very limits of current technology, which isn't anywhere near the case with large sensors).
As we've already shown in a separate article, the a7S's sensor design out-performs its contemporary rivals, even when normalized to the same output size - suggesting it's its very low noise floor, as well as the size, that's allowing it to do so well. This makes the a7S the current camera to beat, when it comes to low light photography. In support of this, the a7S also improves on the low light focus capability of preceding Sony models. Its focusing slows down noticeably but, unlike previous models, will continue to find focus rather than giving up. This can cause problems if your subject moves a lot during focusing but works well with relatively still and high-contrast targets.
The a7S is able to produce impressively clean images up to around ISO 25,600 and even several stops above that become usable with the application of a little luminance noise reduction (which is minimized in this demonstration). As the sensitivity climbs the advantage the Sony offers over its rivals increases. Sensor size alone would lead you to expect the a7S to have a 2EV advantage over the Panasonic but it actually stretches this to nearer. This is to be expected, given how well it performed against its full frame peers in our earlier testing.
Overall image quality
We've had mixed experiences with the JPEG output from cameras that use Sony's Bionz X processor - with the context-sensitive noise reduction appearing a little heavy-handed on the RX100 III and the other a7 models. However, either Sony has adjusted the tuning for the a7S or the lower pixel count means you don't end up seeing the fine-detail that the algorithms tend to have an impact on, we were pretty pleased with the results here.
It's certainly quite possible to get better results out of the Raw files than the JPEG engine can manage, but that's pretty normal. The JPEG engine is good enough that we're disappointed not to have the option to re-process Raw files in-camera, in order to optimize the settings before Wi-Fi-ing images off the camera.
What isn't standard, by any means, is the high ISO performance, which trumps not only the Panasonic but also offers some sort of edge over everything else on the market at present. And this high ISO performance isn't just characterized by usable images at extreme ISO settings but also a retention of Raw processing latitude over a wider range of settings - making the a7S a highly flexible low light performer.
Regardless of the (very) occasional impact of the compression applied to the camera's Raw files, overall the a7S provides an interesting alternative to the a7 in terms of offering some of the best available image quality available in such a small package.
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