The sensor for the A7S III has leaked on the EOSHD Forum, with 4K/60p and an HDR mode in 4K and at 15 megapixels thanks to a Quad Bayer pattern. If you care to take a look at the specs here – you’ll notice a few interesting things. There is only a small megapixel boost from the old 12 megapixel sensor (which rules out 8K) but a completely different architecture with 60 million photosites in a Quad Bayer arrangement, so in this respect the sensor represents a big step up in resolution and colour capture data from a standard bayer sensor.
Due to the Quad Bayer array, images in 4K and stills at 15 megapixels will look crisper and sharper than those from a standard 15 megapixel bayer sensor. So resolution in 4K and in stills mode should benefit from this ‘oversampling’ method, and be a big improvement on the standard 12 megapixel bayer sensor from the old A7S II.
The output in 4K/60p of the sensor is 12bit and is likely to be recorded at 10bit in-camera. Stills shooters are used to 14bit sensor data for the best dynamic range, but most video modes use a 12bit readout to achieve the faster frame rates. The A7S III sensor A/D is upped to 14bit or even 16bit in stills mode according to the data sheet.
The incredibly crisp Quad Bayer image in 4K is actually 4.8K and oversampled from 60 million photosites – so it’ll look closer to 6K in the final 3840 x 2160 file.
Unit cell size is a large 7.52um. The unit cell is a pixel in the final output. As you can see, a “unit” pixel is made up of 4x the photosites compared to a standard bayer pixel.
Interestingly, the sensor hints at some higher frame rates in 4K than 60p.
There is a 3840 x 2160 @ 90fps mode and a letterboxed 3840 x 1346 @ 120fps. Indeed, this mode could have taller pixels rather than a crop, making 4K 120fps possible as a standard recording mode, with the relevant image processing. As the sensor’s vertical resolution goes down, the frame rate increases dramatically, as high as 2000fps. There is a 2K mode at 540fps but at 2394 x 355 it may end up looking rather line-skipped and letter-boxed.
There is no mention of 8K at all in the sensor specs, but there is a 61 megapixel readout mode at low frame rates up to 15fps, most likely for the stills mode of the camera, so Sony can market it as a 61 megapixel camera in keeping with their flagship A7R IV – avoiding the usual trade off between low light and resolution. That is, if they choose to implement this sensor mode at all. Maybe they will, and that’s why they wanted to get the FX9 and A7R IV out of the door first – because this has the potential to cannibalise both for resolution, low light and 4K quality.
Sony say the sensor is aimed at consumer stills camera or camcorders, and not for professional or industrial use. It is a full frame back-illuminated (BSI) design and almost definitely heading for the A7S III sometime between CES in January 2020 and NAB in April 2020.
Worryingly, the Quad Bayer design seems to leave no room for on-chip phase-detect AF or a Dual Pixel AF architecture – at least not in the specs sheet, since there’s absolutely no mention of it. I believe there are smartphone Quad Bayer sensors with on-chip phase-detect autofocus, so it’s curious to see zero mention of it in the specs for this sensor.
