Limiting resolution is easy to determine, but there’s a whole lot more to image sharpness and overall quality than resolution. For example, you can optimize a pinhole camera for maximum resolution, OR you can optimize it for a sharper-looking image!
Check out “The Pinhole Camera Revisited, or, The Revenge of the Simple-Minded Engineer”:
www.biox.kth.se/kjellinternet/Pinhole.pdf
If you want to get into it more, look into Zeiss’ “How to Read MTF Curves”, at:
http://www.zeiss.com/c12567a8003b58b9/Contents-Frame/d9a30a166f326924c125751a004ab770
There are two PDFs there; read the first one (the link at the bottom of the page, not the one in the sidebar), then follow the link to “Part 2” and read the PDF linked at the bottom of that page.
If, after you read them, you still want to fight the pixel-count wars, be my guest… but leave me out of it; I’m more interested in pictures than in pixels!
[With a tip o’ the hat to Don Craig for the Zeiss links, and Gary Powe for the Pinhole PDF.]