Pixel counts are objective. But the problem is that while the pixel count sets the upper bound on the image quality, all cameras don't achieve that upper bound - especially due to the Bayer color filter matrix.
To hit this point home, try taking a photo on a smartphone with a 12-megapixel camera. Now take the same photo on a professional camera, say the Canon EOS R5 with an expensive L-series prime lens. The R5 photo has 45 megapixels, so go and downsample that to 12 megapixels to match the phone photo. The pro camera photo will look much better than the phone photo, thus demonstrating that the phone camera is not maximizing the limits of the 12 MP picture format.
(Also, to make the comparison fairer, take a landscape photo of faraway scenery and buildings so that there are no effects of focus blur from having a large lens aperture. Everything in the photos should be in sharp focus.)
None of this negates the fact that the spatial frequency resolution plots for the lenses, or lenses+sensor combined system would adequately describe these facets under discussion. So what if a manufacturer with their 4X monochromatic pixels that really only constitute just X color pixels, but insinuates the 4X numbers to be the full color pixel count? The spatial angular frequency plots of the compound lens(es)+sensor would quickly become the "figure of merit" to check regarding these facets. Of course nothing would prevent them from stating lies and providing false plots, but then any laboratory could start underwriting (measuring and publishing) such resolution specifications.
I’d be curious re your thoughts on something like a iPhone 16 vs my d6 MkII with a 24-70 or 200 EF lens. The telephoto obviously wins at distance but for family portraits I’ve found I get better photos with the phone and I’m not sure why. I chalk it up to iPhone image processing voodoo vs my setting choice and raw editing skills.
Pixel counts are objective. But the problem is that while the pixel count sets the upper bound on the image quality, all cameras don't achieve that upper bound - especially due to the Bayer color filter matrix.
To hit this point home, try taking a photo on a smartphone with a 12-megapixel camera. Now take the same photo on a professional camera, say the Canon EOS R5 with an expensive L-series prime lens. The R5 photo has 45 megapixels, so go and downsample that to 12 megapixels to match the phone photo. The pro camera photo will look much better than the phone photo, thus demonstrating that the phone camera is not maximizing the limits of the 12 MP picture format.
(Also, to make the comparison fairer, take a landscape photo of faraway scenery and buildings so that there are no effects of focus blur from having a large lens aperture. Everything in the photos should be in sharp focus.)