Dpreview says the A9 speed comes at the cost of over one stop dynamic range compared to the A7rIi

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Dpreview analyzed the dynamic range performance of the Sony a9 and concluded:

We suspect the higher readout speed is leading to greater noise. In other words, it appears this sensor was likely optimized for speed at the expense of low ISO dynamic range.
There is little to no difference in base ISO dynamic range in different drive modes. So the good news is that the drop to 12-bit in continuous drive comes at no cost. The bad news is that the 14-bit Raws aren’t any better than the 12-bit ones.

But it has to be noted that the “cost” is quite small. Still the article is interesting because it explains how to get the best quality out of the Sony A9:

This means that if you’re shooting in conditions demanding high ISO, for any given focal plane exposure you may wish to at least increase in-camera amplification to ISO 640 to get most tones above the noise floor, if your scene demands the extra amplification to get a usable image. Dropping below ISO 640 to preserve highlights, and then raising shadows afterwards, will come at a greater noise cost than, say, Sony’s own a7R II.
Interestingly, this means there’s little advantage to those large (47MB) uncompressed 14-bit Raw files, save for the lack of compression artifacts. In a perfect world, Sony would have offered a 12-bit Raw mode with a lossless compression curve (without that second stage of localized compression that leads to edge artifacts) for smaller file sizes with minimal loss in quality.

Overall, the A9 is a camera made for sports photographers and for those who seek speed. And 1 stop loss of dynamic range will hardly be noticeable or important to them. If Dynamic Range is what you need then buy the A7rII or have the patience to wait for the A7rIII :)

UPDATE: SteveGJ made an interesting comment on Dpreview findings:

There’s a lot of rubbish some people talk about 12 vs 14 bit RAW files. The fact is, by the time you get to about 800 ISO even the best FF cameras only have about 10 bits worth of dynamic range (at the pixel level) so 12 bits are ample as the bottom 2 will just be noise anyway. It will only be at very low ISOs where you might see some difference, and then only if the sensor has low read noise. Of course you get a bit more back when downsizing, but RAW files contain per-pixel data.
On an ISO-invariant sensor you lose one bit of resolution for every doubling of ISO speed anyway, so even 12 bits becomes irrelevant. Of course this camera is clearly not ISO invariant as it’s probably optimised for higher ISOs needed for action photography (the Canon & Nikon equivalents to the same), so it might be that the loss of (bit) resolution doesn’t follow that precise model, but it’s true in general.

Sony A9:
Sony A9 at BHphotoAdorama, Amazon, BestBuy. FocusCamera. Calumet.de. Park UK. Wex UK. Jessops.
Sony A9 with Grip Extension Kit at BHphoto, Adorama and Amazon.
Note: US readers preordering the A9 at BHphoto or Adorama using our links (and our affiliated links only!) have a chance to win a Sony A9.

100-400mm GM:
Sony 100-400mm GM at BHphoto, Amazon, Adorama, BestBuy. Calumet.de. Park UK. Wex UK. Jessops.

Sony A9 accessories:
Sony A9 battery grip at Adorama, Bhphoto, Amazon. Wex UK. Jessops.
Sony A9 Glass screen protector at Adorama, BHphoto, Amazon.
NP-FZ100 battery for the A9 at Adorama, BHphoto, Amazon. Jessops.
GP-EX1 grip at Adorama, BHphoto, Amazon. Wex UK.
FDA-EP18 eye cup at Adorama, BHphoto.
MQZ1 multi battery charger at Adorama, BHphoto, Amazon. Wex UK.
BCQZ1 Z-series Battery Charger at Adorama, BHphoto, Amazon.

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