is there a simple way to detect blur? I want the iPhone to show an alert that the previous photo was blurred - I ended up with a blurred photo of text on a poster as I walked past it.
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)