A few years back (a decade, perhaps?), one of the photo magazines reviewed a
high-resolution Nikon CoolScan scanner. This model had doubled its linear
resolution, going from (about) 2000dpi to 4000dpi. The reveiwer said that,
much to his surprise, he could see a slight improvement in the resolution
and sharpness of the scanned negatives.
This was disheartening. It suggested that a sensor would need an impossibly
(or at least impractically) high number of pixels to equal film resolution.
A 35mm-sized sensor would have to be 4000x6000 pixels... 24Mp!
And that's full-RGB pixels. Most cameras use the Bayer pattern, which only
samples the image. So the total needed to equal the CoolScan's resolution
would be a staggering 72Mp!
In practice, this doesn't seem to be necessary. Given a good lens and
sensor -- and good RAW conversion -- a 12Mp Bayer sensor seems to be
sufficient, even for 16x24 prints. I have a Canon 5D, and it makes gorgeous
photographs, and big enlargements. If you dig through the online reviews,
you'll find a few pro photographers suggesting that the 5D's image quality
is comparable to medium format (4.5x6). How do you get that out of 12Mp? I
have a few theories, but I don't really know.
Popular Photography has shown that good digital cameras exceed the
resolution of Velvia. (I don't know how they'll compare with Kodak's new
ultra-finegram negative material.)
There's a practical limit to the lower size of sensor pixels, determined by
how far you can amplify their output before noise becomes a problem. Barring
some technological breakthrough (which seems unlikely, as we're battling
quantum physics), the limit will likely be reached in about five years.
There is also a speed battle, with camera companies pushing ISOs to 25600.
My guess (and it's wholly speculation) that full-frame Bayer sensors will
top out at around 40Mp, with "acceptably low" noise at ISO 25600.
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