This is a tutorial for Photoshop CC 2015, but earlier versions work just fine.
Frequency separation is useful for portrait editing because it allows you to split your image into two layers. One is the “high frequency” layer that contains the texture of the skin and the fine details. The “low frequency” layer contains brightness information, colors, tones, etc. When you split these into their own separate layers then you’re able to focus your editing on clearing up skin imperfections while keeping the skin texture.
Create the low frequency layer. Duplicate the background layer, label it “Low Frequency”. Then apply a gaussian blur from the menu by going to Filter > Blur > Gaussian Blur
Create the high frequency later. Duplicate the background layer, label it “High Frequency, set the blend mode to Linear Light, then put this layer above Low Frequency, and select it. Now in the Menus, go to Image -> Apply Image….
Set the layer to “Low Frequency”, set scale to 2, then select the settings below depending on your bit depth, which you can check in the menus by seeing what bit depth is selected under Image > Mode >
For 8 bit use the following settings:
- Invert: Unchecked
- Blending: Subtract
- Offset: 128
For 16 bit use the following settings:
- Invert: Checked
- Blending: add
- Offset: 0
For blotchy skin select patches of skin with the lasso tool on the low frequency later, then add a slight gaussian blur to even it out.
For skin imperfections use clone stamping on the high frequency layer while having Sample set to “Current Layer”