Which of course sends us directly to Kieth who has, of course, already investigated the concept and published it ages ago
Evaluating the Canon 5Ds in terms of real world print performance, specifically comparing prints taken with the 11MP 1Ds and 21MP 1Ds mk3
www.northlight-images.co.uk
Editing, upsizing and sharpening part of an old digital photo from a 10MP camera to make an A2 sized print. Image resizing for prints
www.northlight-images.co.uk
A guide to print and image resolution and minimum viewing distances for prints. How much resolution do you need for large prints
www.northlight-images.co.uk
They are very good resources and I do like Keith's reviews and articles as he looks at things from a practical photographer's point of view.
To be honest I think that if everyone's main interest was to print pictures we wouldn't see the same drive for higher and higher mp, but of course generally the complete reverse is true and people are getting larger and larger monitors, with higher and higher resolution screens and want to view their images at full size, and good for them if that is what they enjoy. In fact the old adage about not seeing the wood (forest) for the trees is applicable although now you could say 'not see the wood for the bark on the trees' !
I read people who defend the need for higher and higher mp saying 'why wasn't 8mp enough' and then 10, 12mp etc, 'why did we need 20' and so on. Well there is a perfectly reasonable answer to this; given the physical size that we humans are, and our average vision, 8mp is only enough to print very small at a high dpi, and the limit of the resolution means that if you interpolate up to a greater sized output, although the image may be perfectly 'sharp' detail is lacking. 20mp on the other hand is itself very high resolution, has a native output size of a good medium sized print, say A3 super, and has the resolution (assuming a good file) to be interpolated up much larger, to what we would consider to be a big print and still have a high dpi. So as you go beyond 20mp in FF it is diminishing returns IMO. Certainly this is what I have found in using the 5DS for a few years.
All I can say is that after four years of using 50mp cameras my next one will definitely be a lower mp, somewhere between 20 and 30. Then at least as I upgrade by computer systems I can benefit from faster processing whereas at the moment as I upgrade computers to faster machines I also end up lifting the size of the files and so from a processing speed point of view stand still.
I've been shooting digital since 2005 when the original 5D came out, and there isn't one of my 12mp pictures that I've now thought 'what a pity I didn't take that on a 5DS', at least from a resolution point of view.