Frequency separation

Learning goal

You will be able to correct color and tone irregularities in a face without touching the skin's pored texture. You will explain that the information in a photo sits at two scales: large, soft transitions of color and tone sit in the low frequency, while fine details such as pores, strands of hair and threads of fabric sit in the high frequency. You will set up frequency separation, which splits the photo into these two scales, and choose the separation radius according to how fine the texture is; while you correct redness and dark spots on the low-frequency layer with gentle tone evening, you will leave the texture on the high-frequency layer as it is, or clean up only individual flaws. You will check how excessive tone evening makes a face look plastic, why a person's permanent color features shouldn't be erased with this technique, and how a correction made on the low-frequency layer can shift color on different skin tones.

The question

This time the drummer opens their own close-up portrait: a frame for an album promotion, their face filling the frame, their eyes looking straight into the lens. The light is good, the expression exactly what they wanted. But looking closely, they see a pink ring at the base of the sides of their nose, three or four brown dots along their jawline, and on their forehead the uneven tones a tired night left behind.

First they try the healing brush. It erases the spots, but where the brush passes, the skin's pores disappear too; the face turns into a smooth patch of wax compared with its surroundings. Then they try blurring; the redness softens, but this time the whole face looks like a plastic mask, with neither pores nor the roots of their stubble left. There must be another way to remove the spots and keep the pores in place. How do you correct skin while keeping its texture?

Context

In the cleanup lesson you saw how to erase individual flaws, and in the dodge and burn lesson how to arrange light and shape without touching texture. A gap remains between these two methods: color and tone irregularities spread over a wide area of a face. Erasing them with the healing brush erases the texture too, and blurring makes the whole face plastic; you will remember the waxy effect that noise reduction creates when overdone in basic editing (Lesson 17.6).

Frequency separation solves this problem by splitting the photo into two separate layers: color and tone sit in one, texture in the other. This way, while you touch one, the other stays in place. It is a powerful technique, and in the wrong hands one of the techniques that produces the most "plastic faces"; that is why you need to know both how it is set up and where to stop.

Simple explanation

What does frequency mean?

You can split the information in a photo in two according to the scale of the detail. Low frequency is the photo's large, soft transitions of color and tone: the redness of a cheek, a forehead slowly darkening from light to shadow, the overall color of a skin. High frequency, on the other hand, is small, sharp detail: pores, fine wrinkles, eyebrow and beard hairs, the threads of fabric. When you blur a photo heavily, only the low frequency remains; everything lost to blurring is high frequency.

Frequency separation is a retouching technique that splits a photo into these two scales and keeps each one on a separate layer. The low-frequency layer is a blurred copy of the photo that carries only color and tone. The high-frequency layer, on the other hand, is the difference between the original photo and this blurred copy; it carries only texture and fine detail, and looked at on its own it appears like an embossing on a gray surface. When the two layers are placed one on top of the other with a suitable blend mode, if no correction has been made, an exact copy of the original photo is obtained. This is the simplest way to verify the setup: you turn on both layers together and compare them with the original photo; you shouldn't see any difference.

How is the separation radius chosen?

The separation radius is the amount of blurring used while creating the low-frequency layer, and it decides which detail falls onto which layer. If the radius is small, even fine pores stay in the low frequency and the texture is split across the two layers; if the radius is large, large shadows that belong to the face's shape are also carried into the high frequency. The right radius depends on the photo's resolution and on how much the face fills the frame; that is why there is no single number.

The practical measure is this: while blurring the low-frequency layer you enlarge the face and watch; you stop at the moment when the pores and fine lines have fully disappeared, but the face's large shapes, the nose, the eye sockets and the cheekbones, can still be made out. This moment is the moment when texture separates into the high frequency, and color and tone into the low frequency. In a close-up portrait this radius is larger than in a full-length portrait taken from a distance.

What is done on each layer?

Tone evening is gently balancing color and tone irregularities on the low-frequency layer. On this layer, with a soft-edged, low-flow brush, you can pick the healthy tone around a red area and paint lightly over the redness, or bring tones closer together with a very gentle healing. Since none of the texture is on this layer, however much you paint, the pores stay in place. Doing the tone evening on a separate empty layer and adjusting its opacity when the work is done preserves undoability.

On the high-frequency layer, on the other hand, most of the time nothing is done; the texture should already stay as it is. Only if there is a flaw in the texture itself, for example a single coarse hair or a line left by dust, do you erase it with the tools from the cleanup lesson, taking the source from nearby, similar texture. Since there is no color on this layer, the patch's color never stays mismatched.

Where do you stop?

The biggest danger of frequency separation is flattening the low-frequency layer too much. The face's natural color transitions, the warmth of the cheeks, the small tone differences between the tip of the nose and the forehead give a face its liveliness. Evening them all to the same tone turns the face into a painted mask, even if the pores stay in place. The measure is reducing the irregularity, not eliminating it; pulling back the opacity of the tone evening layer a little brings naturalness back.

The permanent features of skin color shouldn't be erased with this technique either. Freckles, birthmarks, color differences in the skin such as vitiligo, or scars are, following the distinction in the cleanup lesson, part of the person; that they "can be evened out" on the low-frequency layer doesn't mean they should be. You need to ask the person about the difference between a temporary redness or a trace of tiredness and a permanent feature.

Because corrections made on the low-frequency layer change color, they can lead to shifts in skin. On dark skin, if a wrong tone is picked while painting dark spots with a tone chosen from the surroundings, the area can look gray, ashy or lighter than its surroundings; you must always pick the tone from the skin right next to the spot, in the same light. On light skin, suppressing redness too much can make the face look pale and ill; the natural warmth of the cheeks and lips should be kept. In both cases you need to turn on both layers together, compare the face from a distance and up close, and check that the color stays close to the person themselves.

Visual evidence

The drummer makes two copies of their close-up portrait. They blur the lower one while enlarging their face; they stop when the pores on their forehead and the roots of their stubble have fully disappeared, but the shape of their nose and eye sockets can still be made out. They separate this blurred layer from the upper copy; the upper layer carries the pores, eyebrow hairs and stubble that look like an embossing on a gray surface. When they turn on both layers together, there is no difference between them and the original photo.

Right above the low frequency they add a separate, empty layer, pick the tone of the skin right next to the redness around their nose, and paint over the redness a few times with a low-flow, soft brush. The redness softens, the pores stay as they are. They also lighten the dark spots on their chin, taking the tone for each one from the skin right next to it in the same light. The small birthmark on their cheek, on the other hand, they don't touch at all. When they lower the opacity of the tone evening layer to sixty percent, the face is still lively; the warmth of the cheeks and the forehead's descent from light to shadow are in place. Their dark skin is in its own brown, their pores and stubble sharp.

Common mistakes

Summary

Frequency separation splits a photo into a low-frequency layer that carries the large transitions of color and tone and a high-frequency layer that carries pores and fine details; together the two layers give the original photo exactly. The separation radius is chosen at the moment when the pores disappear and the face's large shapes can still be made out, and it changes with the photo's resolution; tone evening is done in the low frequency, and in the high frequency only the texture's own flaws are cleaned up. If the low frequency is flattened too much the face turns into a mask, permanent color features aren't erased, and so that the tone doesn't shift on skin, it is picked from the skin right next to the flaw, in the same light.

Mini assignment

In this assignment you will correct a close-up portrait with frequency separation; if possible use your own face or the face of someone whose permission you have.

1. Make two copies of the photo; blur the lower one until the pores disappear and the face's shape can still be made out, and note the radius you used.
2. Create the high frequency on the upper layer; verify the setup by turning on both layers together and comparing them with the original photo.
3. On an empty layer above the low-frequency layer, gently even out a redness or a dark spot by picking the tone of the skin right next to it; compare the before and after states.
4. Deliberately flatten the same area completely once and see the difference; then find a natural measure by lowering the opacity of the tone evening layer. Ask the person whether there is a feature they don't want erased.

Check yourself

• Can you explain what the low and high frequencies carry in a photo?
• Were you able to choose the separation radius at the moment when the pores disappeared but the face's shape remained?
• While evening tones, did you check that the tone you picked came from the skin next to the flaw in the same light and that the skin's color didn't shift?

Glossary

Mini quiz

1. In frequency separation, which layer are a face's pores and beard hairs on?

• A) On the low-frequency layer
• B) On the high-frequency layer
• C) Equally on both layers
• D) On neither

2. How do you verify that the frequency separation setup has been done correctly?

• A) By deleting the high-frequency layer
• B) By blurring the low-frequency layer even more
• C) By changing the blend mode
• D) By turning on both layers together and seeing that there is no difference between them and the original photo

3. At what moment is the separation radius chosen?

• A) When the pores have fully disappeared but the face's large shapes can still be made out
• B) When the face is completely blurred
• C) When the pores first start to blur
• D) With the same number in every photo

4. After the spots on a dark-skinned person's chin are painted on the low-frequency layer, the area looks gray and lighter than its surroundings. Which is the most likely cause and fix?

• A) The radius is too small; enlarge the radius
• B) The high-frequency layer is broken; delete it
• C) The picked tone was taken from a different area; pick the tone again from the skin right next to the spot, in the same light
• D) The blend mode is wrong; set it to normal

Answers

1. B — Fine, sharp details are lost to blurring; that is why they sit in the high frequency.
2. D — While no correction has been made, the two layers together should give the original photo exactly.
3. A — This moment is the moment when texture separates into the high frequency, and color and tone into the low frequency.
4. C — The low frequency carries color; a tone taken from the wrong place doesn't match its surroundings.

Summary card

• Low frequency is color and tone, high frequency is texture; the two layers together are the original photo.
• Separation radius: the pores go, the face's shape stays.
• Even tones in the low frequency, clean up only the texture's flaws in the high frequency.
• Don't eliminate irregularity, reduce it; measure with opacity.
• Don't erase permanent color features; pick the tone from the skin next to the flaw, in the same light.

Bridge to the next lesson

The close-up portrait corrected with frequency separation looked natural to the drummer, but the band's social media manager also edited the photo their own way: the face completely smooth, the eyes large and shiny, the chin slightly slimmed, the skin lightened by a tone. The drummer had trouble recognizing themselves in the photo. With the two versions side by side, the same question came to both of them: where does retouching stop improving a photo and start changing a person? How much retouching is too much? The next lesson deals with the limits of improving without changing the person and the "plastic face" trap.