由于衍射的存在, 像素数目有个"优化"值.
To quote:
"Consider a 35mm system with a lens at f/11. At best, the maximum resolution you will get is equivalent to 16 MP, even if your camera has 22 or 25 MP"
详细的计算与论述在这里:
http://luminous-landscape.com/tutorials/resolution.shtml