Frequency separation is a powerful skin editing technique that can smooth skin, minimise blemishes, reduce under eye circles and blend skin tones, without losing skin texture. And the mixer brush tool works so well with this technique!
But how does FS work?
In any image there are low frequencies and high frequencies. The low frequencies are skin tones and colours, while high frequencies are textures. The purpose then of frequency separation is to separate the high from the low so that you can work on each element individually. To smooth skin and even out colour and skin tones you can work on the low frequency layer without making any changes to the texture (high frequency) on the skin.
For the image below, my first step was to make small adjustments to my highlights and shadows in ACR. Then opening the image in Photoshop I used the healing brush tool to remove obvious blemishes and the patch tool to reduce under eye circles and areas of redness. Only when you have made these adjustments, should you begin with frequency separation.
When you begin, select the low frequency colour layer and apply the brush settings as shown above. Use a small brush and start to blend, being careful to move in the direction of the skin texture. Zoom in to see the direction of the area you are about to work on. I find using small circular brush strokes works best for me. This method can be a little time consuming, even more so when you are learning, but once you get the hang of it, the results are so worth it!