How to do frequency separation retouching

Frequency separation retouching starts with one command in Paint by Zaweba: Layer > Frequency Separation. It splits the picture into a group of two layers, a Low layer holding the colour and tone and a High layer holding the fine texture in Linear Light mode, which recombine exactly into the original. You then even out blotchy colour on the Low layer and clean up texture on the High layer without one spoiling the other.

It's the method retouchers use to make skin look smooth while keeping its pores, instead of the plastic look you get from just blurring a face.

How the split works

Any picture can be thought of as broad changes of colour and light, plus fine detail laid over them. The Low layer is a blurred version holding the broad changes. The High layer holds the difference between the original and that blur, which is all the fine detail: pores, hairs, fabric weave. Because the High layer is set to Linear Light, the two together give back the original exactly. Nothing changes until you edit one of them.

Paint offers three ways to make the Low layer: Gaussian, surface or bilateral. Gaussian is the classic choice. Surface and bilateral blur while keeping edges, which can stop halos at the edge of a jaw or nose.

Step by step with frequency separation

Open the portrait and finish any basic corrections first, such as exposure and white balance. Choose Layer > Frequency Separation and pick the blur type and radius. The radius sets the dividing line. A good starting point is the smallest radius at which the pores just disappear from the Low layer when you look at it alone.

On the Low layer, work on colour and tone. Use a soft brush at low opacity, sampling nearby skin, to even out red patches and shadows under the eyes. You can also select a blotchy area and blur it a little more. Because this layer has no texture, you can't smear pores by accident.

On the High layer, work on texture. Use the clone or healing tools to remove spots, stray hairs and lines while copying real texture from nearby. Because this layer has no colour, you won't drag a wrong skin tone across the cheek.

Getting a natural result with frequency separation

Restraint is everything. Skin has variation in colour, and a face with every blush removed looks lifeless. Keep the Low layer edits gentle, and lower the opacity of your work if it starts to look painted.

Leave the features alone. Eyes, lips, eyebrows and hair should keep their full detail, so avoid them on both layers or mask the work away from them.

Zoom out regularly. Retouching at 300 percent makes every pore look like a crater, and you'll overdo it. Judge the result at the size the picture will actually be seen.

Working alongside other tools

For simple blemishes, the healing tools on a normal layer are often quicker, and you may not need frequency separation at all. Dodge and Burn are useful afterwards to shape light on the face. Keep everything in the .ptx so the group stays editable.

Paint by Zaweba is free software for Windows that runs offline, which is handy for retouching private portraits.