An interesting - and somewhat intuitive - observation is that the sharper the focus the higher the spatial frequencies go (this is pretty obvious when viewing a

An interesting - and somewhat intuitive - observation is that the sharper the focus the higher the spatial frequencies go (this is pretty obvious when viewing a composite video signal on an oscilloscope while adjusting the camera focus - the peaks are 'pointiest' at best focus). In the 1D case, consider a dashed line. With perfect focus this is equivalent to a square wave (which is a sum of all the odd harmonics of the fundamental frequency). Loosely, the blurrier the image of the dashes the lower the frequency content - eventually blurring to 50% grey which is essentially DC.