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We've just had the chance to put a production Sony a9 III through our studio scene. The question we most wanted to answer was: is there any image quality cost to adopting a global shutter sensor? The short answer is: yes.
Global shutter sensors have been available and used in industrial settings for some time now, but haven't made their way across to photography because the more complex design meant their image quality wasn't a match for the best progressive-scan CMOS designs. Sony said the a9 III's Stacked CMOS design overcame any compromise in ISO or dynamic range. This doesn't appear to be the case.
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However, in the context of a professional sports photography, the compromises that have been made may well make sense, in return for the sheer speed of capture the a9 III is capable of, both in terms of burst rate and its fast, distortion-free shutter.
The most immediate difference in capability is that the a9 III has a base ISO of 250. This means that you can't give it as much light as its peers with base ISOs of 100 or lower. This is not necessarily an issue for sports photography, where maintaining a high shutter speed is much more important than the need to optimize image quality by staying at a low ISO.
Has the studio scene changed?The a9 III is one of the first cameras we've shot since setting up our studio scene at our new location. Our a7CR images raised concerns about how consistent the results are between the new installation and our previous setup. In response to these concerns, we re-shot the Sony a7R V (whose higher resolution viewfinder makes it much easier to fine-focus than the a7CR) and checked the Raw values against the photos taken in the old studio. With some slight adjustment of the lights, we reduced the existing 0.08EV discrepancy down to 0.02EV difference for the grey patches we use for noise assessment. We wanted to make sure that both we and our audience could have complete faith in the consistency of the test scene before testing the a9 III. The files shot on Jan 2nd 2024 and the original versions, as featured in the comparison tool, shot on Nov 11 2022, can be downloaded here. |
What might be of more concern to sports shooters is that the high ISO performance appears to be as much as one stop noisier than its full-frame rivals, especially as you reach its highest ISO settings . There's a noticeable softness to the 'grain' pattern in the a9 III's images too, which we suspect is the result of noise reduction being applied in the Raws.
This is in line with what we expected. Essentially the a9 III's sensor works by having two photodiodes at each pixel: one to capture the light, initially, and the second to act as a holding buffer, that allows all the pixels to be read-out simultaneously. This design effectively halves each pixel's capacity for light, which explains the elevated base ISO and the decreased noise performance, which brings it closer into line with the performance of APS-C cameras. In addition, the complexity of the design means we don't get the dual conversion gain circuitry that helps improve high ISO performance on other recent cameras.
How does the a9 III's dynamic range compare?
Just as staying at low ISOs is rarely critical for sports, nor is maximizing dynamic range for a discipline that generally shoots JPEGs for immediate delivery, with no time to exploit extra DR during careful processing as, for instance, landscape shooters might.
The sensor's reduced capacity for light has an impact on dynamic range, since the entire image becomes noisier, but we should be careful not to double-count this by interpreting it as a separate dynamic range cost. At its launch, Sony told us the a9 III has dynamic range comparable with previous models, and our measurements show that it is comparable with cameras when operating at ISO 250. Notably most other cameras can operate at lower ISOs than this, and hence have a higher maximum dynamic range than the a9 III.
When compared, the Sony a9 II, if anything, shows more noise if shot at its ISO 200 setting when brightened, than the a9 III. The a9 III's smaller photodiodes mean there's more photon shot noise in the ISO 6400 shots (simply because the Mk III captured less light), but if you try brightening the low ISO files there doesn't appear to be an additional (electronic) read noise cost. It's the same story if you try to reduce exposure at base ISO and brighten: the a9 III is a little behind the a9 II because its base ISO is higher, but there's not a big difference in additional noise if you compare similar exposures (where photon shot noise would be similar so differences caused by read noise would become apparent).
Summary
Examining the a9 III's images shows everything that you'd expect from it having a reduced capacity for light. The higher base ISO isn't inherently a problem for sports shooters, so it's simply a question of whether the noise penalty is worthwhile for all the things that super-fast 120fps shooting and global shutter bring. That's something we'll consider in more depth in our final review.
But what does this trade-off mean beyond the pro sports market? Our tests show that this sensor's performance comes with an image quality hit that might make less sense for general photography. Furthermore, this cost of up to a stop of image quality in return for added performance is likely to make global shutter less appealing in the smaller APS-C and Four Thirds formats, which don't have the luxury of so much IQ to give up.
Overall, the a9 III still looks promising, for its intended purpose, but it shouldn't be assumed to herald the future of cameras as a whole.
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