Benefits of frequency separation...

Found this in the forums over on ModelMayhem.com. The question was presented where the OP asked what were the benefits of frequency separation. One of my heroes replied:

Easier to remove blemishes with clone/heal.
Easier to fix uneven skin tones or even change skin tones completely.
It allows for other methods of sharpening.
Much easier to repair areas that were blown out/under exposed.

What he/she is talking about in that tutorial, though, is more of a band pass technique. Think of it like this.....

Blur/Low Frequency Bandpass HF
<--------------------------------------><--><--------->

Now, suppose there are skin imperfections that would fall in the radius/spatial frequency range of the Bandpass section. This is the "Middle Layer" he refers to. You could spend your time cloning/healing if you want. However, if you separate out this frequency properly, you can simply delete it. What you'd be left with is the following...

Blur/Low Frequency HF
<-------------------------------------------><--------->

This isn't an exact representation of what happens, but you effectively remove the information in the image that contains the unwanted skin imperfections. This leaves the nice tones from the LF layer and the good details from the HF layer.

Hope that helps some.

Click here for link to thread. There's a cool little video that explains all of this (that I didn't watch) as well.