Frequency separation helps make retouching a portrait easier in Photoshop. Frequency separation works because it allows you to separate the texture of an image from the tone and color. When retouching you can focus on just the texture or the skin beneath it.
It sounds quite complex but in practice it is quite simple. If you want to work with texture, choose the high frequency layer, if you want to work with skin choose the low frequency layer.
Getting Started with Frequency Separation
To get your image set up for frequency separation you will have to make 2 copies of your background layer. If you are not starting from the beginning, add a Stamp Visible layer over all layers by typing CTRL+ALT+OPTN+E. One of these copies will be for your low frequency (skin) and one will be for your high frequency (texture).
First apply a Gaussian Blur to the lower (low frequency) layer. Choose a radius that blurs out the skin texture, but don’t go too far as to make the face unrecognizable.
Next choose your high frequency layer and go to Image, Apply Image. Here you will want to se the Layer: to the low frequency layer. For blending choose Subtract and for Scale:2, Offset:128. These will be your settings every time, no need to change them.
Now you should have one layer with a blur (low frequency) and a layer above it that looks mostly gray (high frequency). Select the high frequency layer and change the blending mode to “Linear Light”.