And why would that be? I don't understand.
BSI eliminates the wiring that is blocking a part of the light in conventional sensors. As there has to be more of this wiring on a higher resolution sensor, it actually should reduce the difference between high end low resolution even more in terms of noise - and the difference is already negligible in my eyes:
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And DR depends so heavily on noise. I know a single large pixel has higher DR than a single small one - but we are usually care about images, and therefore average multiple pixel together to compare image sections of the same area. Which results in the same light and noise per area, so where is the advantage for lower resolution sensors coming from?
To be clear I do expect Canon to improve both in terms of DR and low light performance with this new sensor. It marks a pretty strong shift in their manufacturing technology after all. I just don't see that improvement being related to the resolution and fully expect the upcoming, higher resolution bodies (R5s, R1?) to outperform it.