Look at the frequency spectrum. If you consider focus, or "blurness" a parameter that varies, the value where the picture is "least blurry" or "most in focus"

Look at the frequency spectrum. If you consider focus, or "blurness" a parameter that varies, the value where the picture is "least blurry" or "most in focus" tends to be a local maxima of the power of the higher frequency components. When the picture is crisper edges will be more pronounced, which is high(er) frequency information. So the high frequency power would be at a local maxima. (If that doesn't make sense yet, try imagining the opposite :D When a picture is the "most blurred" or "most out of focus" it looks like a soft colorfield. low frequency :D)