This is a very useful method to even out skin tones and remove blemishes whilst retaining skin texture as much as possible. I’ve used other techniques in the past (inverse high filter passes, gaussian blur layers and various levels of masking) but this method seems to deliver the best results and minimises the risk of over smoothing the skin.
It’s fairly straight forward to do, and you can record an action to cover the initial setup steps. The process involves creating 2 separate layers from your background image. One contains the colour tone information for the image (and you can work on this to even out the skin tones as much as possible). The second layer holds all the texture information for the image and you can edit this layer to remove skill blemishes, pock marks etc. without altering the colouring (often an issue with using the healing brush tool on a single image layer).
1) Ensure your image is in 16 bit mode: Image > Mode > 16 Bits/Channel
2) Create 2 copies of your background layer. Rename the bottom layer Low Frequency and the top one High Frequency
3) Select the High Frequency layer and go to Image > Apply Image. Set the drop down box to be the Low Frequency layer, Channel to be RGB and check the Invert option. Next set the Blending mode to be Add, keep the opacity at 100% and increase the scale to 2. The settings should be as below. Click OK
4. Next select the low frequency layer, and select Filter > Blur > Surface Blur. The settings here will vary for different images. The idea is to create a layer with no detail, just colour information, so amend the settings until all skin detail (pores etc.) is lost. (In the below image the settings are too high, I went for a radius of about 15 and a threshold of 10 in the end)
5) Group the layers together in a folder and rename it Frequency Separation. You should now have something smilar to the below:
Lets take a closer look at these layers (you can use the eye icon next to each layer to show or hide them in turn).
The low frequency layer contains a “blurred” image, just holding colour information. You can do further smoothing and blending on this layer to even the skin tones further, reduce shiny highlights etc. (I usually make a further copy of this layer to work from).
With this particular image I did some further blending (clone tool, blur tool) to lighten the dark skin under the eyes. The High Frequency layer shows the fine detail (similar to a high pass filter) and zooming in closer you can begin to see the skin texture along with any blemishes.
This is where you can use the healing brush to remove any blemishes, or reduce heavy lines. The Healing Brush Tool or Spot Healing Tool sample pixels from the surrounding area, and this could odinarily introduce patches of discolouration. By doing the healing work on this “texture” layer, it prevents this issue as there is no colour information included (that’s all on the Low Frequency layer). Again I usually make a copy of the High Frequency layer and work on that (so it’s easy to recover from any mistakes).
Here’s a comparison of before and after using this techinque: