Frequency separation is a technique for separating the part of the image with a lot of detail such as skin texture (High frequency) from that without much detail but with such things as skin colour and tone information (Low frequency). This gives a Low Frequency Layer and a High Frequency Layer. When this is done properly the two frequency separations look identical with the original image when viewed together, however, they have the great advantage of being able to be edited separately, which makes many editing tasks much simpler. For example the skin colour can be smoothed out without affecting the skin texture.
It is good for correcting shiny skin as dodge and burn makes the highlights grey.
The technique of Frequency Separation is different for 8 bit and 16 bit images.
Frequency Separation can be done manually but is best used as a Photoshop Action which produces the required layers automatically with the High Frequency Layer on top, then a blank Adjustment Layer, and a Low Frequency Layer below that.
Once the new layers are produced you can not edit the layers below as these layers are not transparent.
Using A Frequency Separation Action
Choose the correct action for the bit depth of 8 or 16 bit.
Adjust the radius setting of the Gaussian Blur until the skin texture can no longer bee seen and then just a little more but not too much.
Editing the Low Frequency (Colour) Layer
The Low Frequency (Colour) layer is where you do your large scale retouching to smooth out uneven tones and colours.
Edit on a Blank Layer above the Low Frequency Layer, so the editing is non destructive. Have the selection mode set to Current and Below.
You can have the High Frequency detail layer turned on or off, whatever you find gives you the best visualisation of the final effect.
The Brush Tool can be used by Sampling from a nearly area and lightly painting over areas to be smoothed with an Opacity of 100% and a Flow of 1-5% for a Mouse or 10-20% for a Pen.
A Gaussian Blur can be used to soften hard transitions in a larger area but this cannot be used on a blank layer, and so should be used on a Stamped Layer which makes a single layer copy of all the Layers but having the High Frequency Layer turned off. This layer should be positioned just below the High Frequency Layer. Use a soft edged Lasso Tool to select the area to work on and then apply a Gaussian Blur to that and adjust the radius to give the best effect.
Editing the High Frequency (Detail) Layer
The High Frequency (Detail) layer is where you do your small scale retouching of uneven textures and blemishes.
Make sure that any editing you do only samples from this layer because if your sample comes from other layers as well the unusual blending mode of Linear Light will produce strange effects. This means that the editing is destructive.
The Healing Brush Tool or Clone Stamp Tool can be used to remove blemishes. You may need to do some correction on the Low Frequency (Colour) Layer as well if the colour still shows to be uneven.
How Retouchers Use This Frequency Separation
Pratik Naik only uses this for final small corrections of skin tones, transitions, lighting and makeup, after using the healing brush, clone stamp, and dodge and burn. His action uses a fixed 3 pixel blur, which he finds works for most images. He feels that this technique is over used and produces artificial looking results.
Advanced Texture Cleanup - Software to go beyond the limits of normal frequency separation and use it more universally. rather than just on skin. This process solves the halo issues which normally limit this process to just skin retouching. $129.