Bill Claff has posted dynamic range measurements of the 1D X Mark III and they show meaningful improvement at low ISOs compared to the Mark II, and a slight improvement over the 5D4/R sensor. High ISO performance is roughly the same, purely numerically, but it's likely that there's further improvement in noise quality (fineness and uniformity) like in the 90D/M6II sensor. When it comes to the competitors, the D5 is still a high-ISO king, although at the expense of a major, roughly 2 EV, disadvantage at ISO 100. The D850 is all but equal to the 1DX3, except for its native base ISO of 64, as are the Z6 and Z7. The α9 Mark II sensor has a tiny advantage at medium-to-high ISOs, but loses to the 1DX3 in the lower range. It also drops to 12-bit processing whenever you so much as look at it funnily, so there's that. As for the α7R Mark IV, its performance is again practically identical to the new 1DX. All in all, this seems to be a great sensor and should bode well for the R5 and R6, too!
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