(not to beat a dead horse...)
Digital Camera resolution is limited by Sensor Size, Pixel Size, Lens Diffraction, ISO Sensitivity, and the IR/moire filter at the Sensor and Bayer filter in the software. However the two critical limiting factors are Pixel Size and Lens Diffraction, due to the laws of optical physics. Diffraction causes resolution to degrade for pixels smaller than 6 micron and apertures smaller than f4.
For practical purposes in print, 6 micron pixel size leads to 12 Mpix cameras at crop frame, 18 Mpix (M9) to 24 Mpix (Sony A900) at full Frame, and 40 Mpix at 645 format. Images on the Web are quite fine using 2 micron pixel-size P&S or cell phones.Film cameras face approximately the same resolution limitations, with grain size in the 2-9 micron range. To complete the picture, the Nikon 9000ED scanner which scans at 4000 dpi works out to 6.4 micron; scanning at 140 dpmm is 7.1 micron.
My D-90 has 4288x2848 5.5 micron pixels. In terms of printing, 300dpi gives me 9.5x14.3 inch (native resolution) landscapes on a Fuji Frontier laserjet. In practice, I can get good prints up to 12x18 using up-rezzing, noise reduction and sharpening, because most people don't walk up to my wall mounts with a magnifying glass.
In terms of ultimate precision, my 5.5 micron pixels already are "blurred" or averaged, due to diffraction, moire filtering, Bayer filter, etc. Of course, my technique is never less than optimal!
I'm sure this isn't new information for anyone, but I'd never framed the problem for myself so concisely. It makes my decision matrix for choosing camera format straightforward: - To print full resolution at 12x18, I need FF.
- To print full resolution at 18x24, I need 645 format.
- To shoot darker scenes, I need a camera with larger pixel size
- To maximize my resolution I need to reduce diffraction, which means using apertures at or larger than f4
- To get more depth of field, I need to use a smaller aperture, which hurts diffraction which means I need larger pixels.
In summary, technology marches, but physics rules. We aren't going to get much higher resolution cameras than 12 Mpix crop-frame or 24 Mpix FF because 6 microns is the limit given diffraction at the f4 aperture. On the upside, technology could give us better ISO (dark performance), or cheaper sensors.
Roger Clark at
http://clarkvision.com provides a long, detailed discussion of "Digital Cameras: Does Pixel Size Matter?". But after reading all the spilled pixels, the resolution question is best illustrated by
one picture showing Diffraction Effects vs pixel size for various apertures and different cameras.