Engineering Tests by Jim Kasson show Sony a7R IV has Dual Native ISOs of 100 & 320 and none of the PDAF Pixel Banding that plagues Fuji GFX 100 and Nikon Z7 cameras.
Jim notes conversion gain changes at the transition from ISO 250 to ISO 320, which is lower than Fuji GFX 100 and Phase One IQ4 150 MP, which use the same 3.76 um pixel pitch BSI Sony sensors. This jump in dynamic range indicates a7R IV has dual native ISOs of 100 & 320, which means is that you’ll actually have more dynamic range when shooting at ISO 320 than you would from ISO 160 to 250.
Please note this is purely a comparison of dynamic range – not noise – though noise is extremely low across this entire range. It’s simply good to know the “sweet spots” for any sensor. For instance if you were shooting a7R IV at ISO 250, you could actually get better results bumping the ISO to 320.
By comparison, Sony a7R III has dual native ISOs of 100 & 640.
In case you’re wondering why the figures here differ from Sony’s published 15-Stop Dynamic Range, Jim explains that EDR is a per-pixel measurement that is not normalized by the number of pixels in the image height.
The even better news is that a7R IV tests show periodic spiking in the spectra of the images that would indicate PDAF-striping “fixes” that cause banding in the Fuju GFX 100 and Nikon Z7 cameras. Quoting Jim, “From this data, I’m surmising that the a7RIV wont have the same PDAF banding the plagues the GXF 100 and the Nikon Z cameras.”
Jim also notes:
• There is low-pass filtering on all channels above ISO 12800.
• There is low-pass filtering on the blue channel; probably to interpolate over the PDAF pixels.