Anyone with expertise on this subject?
Can binning and BSI produce both increased light sensitivity and increased resolution?
I am guessing the in-camera computer could be set to join or read or "bin" a number of pixels at high ISO settings to produce increased light sensitivity (say 4 pixels binned in a 24 megapixel sensor means essentially a 6 megapixel sensor at high ISO) and to unjoin or read separately or not "bin" each of the 24 mega pixels to produce increased resolution at low ISOs. Sounds like a fantastic idea to me if it works and why wouldn't it?
See these comments by Thom Hogan:
When I wrote earlier that the D400 would be 24mp, I got a lot of emails asking if I had written the number wrong (or if I was just plain crazy). Neither. First, we know that a 24mp DX sensor exists (or is about to exist in production form, from Sony). Second, back in 2003 I pointed out that the math said we'd get to about 24mp on DX before we exhausted the easy-to-see gains and started outshooting the best existing lenses. Third, at 16mp the Nikon would be trailing its two primary competitors in that market. Fourth, there's the "it's a mini D3x" notion that many will have when they see the D400. So, yes, we'll go there. 24mp is a done deal at some day in the future, so if that Sony sensor is good, the future is just about here. (Beyond 24mp I think things get much more fuzzy, and that's not just a pun on diffraction impacts.)
Now for the part that, I'm guessing, could be Nikon's surprise in August: binning. Binning isn't new. The D1 was a 10.4mp sensor that was permanently binned (four underlying photosites to one pixel). Nikon has played with binning since then, too, with the oddly binned D1x (two side-by-side photosites to one pixel). The answer to the "why 24mp question" suddenly becomes simple if you add binning to the mix: you potentially get a D400h and D400x all in one package. I do know that Nikon and Sony have been working on binning recently. That would make some sense for both stills and video and could lead to a jack-of-all-trades DX body that excels at everything: low noise, high resolution, video without artifacting, etc. And it would explain pursuing 24mp in DX and 36mp in FX.
But I'm only guessing at the binning thing. Without binning, and especially without BSI, we just have more pixels which means more data and power lines cluttering the sensor, meaning less light efficiency. Thus, even with the normal expected gains of a new generation of sensors, 24mp wouldn't really get us much further than the 16mp DX capabilities we already have, if any. Plus 24mp by itself really is pushing the full recording of diffraction into most image data.