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Don't forget about diffraction of the aperture. In the tests here you will see that with higher density sensors, performance of top lenses in the center already starts to fall at f4 or f5.6.       

What would be the optimal pixel density on a sensor when the lens has f2.8? or f2.0?

Hard to tell. The diffraction of light is linked to the wavelength of the light. Since you never deal with monochromatic light, but resolution figures usually are given for monochromatic light, real world resolution will be lower than published data.

 

For e-line (wavelength of the light: 546.1nm, a green) diffraction limited resolution is:

f1.0 : 1501 lines/mm

f1.2 : 1251 lines/mm

f1.4 : 1072 lines/mm

f2 : 750 lines/mm

f2.8 : 536 lines/mm

f4 : 375 lines/mm

f5.6 : 268 lines/mm

f8: 188 lines/mm

f11: 136 lines/mm

f16 : 94 lines/mm

f22 : 68 lines/mm

(source: Nikon Ultra Micro Nikkor lenses brochure)

 

This is for diffraction limited lenses... So, lenses whose sharpness is limited by diffraction. In reality one does not find diffraction limited f2.8 or f2 lenses for DSLRs (maybe there are one or two exceptions, I do not know). Again, the numbers are for monochromatic light (e-line). Red has a longer wave length (around 700nm), and will cause resolution limited diffraction sooner (for f2 : 585 lines / mm, for f2.8 : 418 lines/mm (source: [/url]

 

For the different wave lengths for (monochromatic) blue, green and red you can find the optimal MP-count for different sized sensors here too: [url=http://www.luminous-landscape.com/tutorials/resolution.shtml]http://www.luminous-landscape.com/tutorials/resolution.shtml

If you take the green-yellow wavelength (550nm) they suggest as reference, they give for f2 on APS-C 209MP and for f2.8 on APS-C 107MP. But this still represents monochromatic light, and yellow and red wavelengths will lower diffraction limit more.