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As for aperture, diffraction, and DoF, it will all be a compromise, so play around and see what the limits are of each body/lens combination you have. I can use a much smaller aperture with my 10D (6 megapixels) than I can with my 1Ds2 (17 megapixels), but that's the way it is. You should check out a DoF calculator (several free online). Also, google "hyperfocal distance", which is useful for landscape photography.