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“Dammit, Jim, I’m a designer, not a rocket scientist!” I hear you cry. Well, before you browse away in search of something less scary, this isn’t hard. In fact, it will make your retouching life a lot easier. The best thing is, it uses features that have been in Photoshop for years.
First things first: what the heck are we talking about?

Frequency is Image-Geek for the levels of detail in an image. High Frequency information is the fine detail (think grass, hair, grains of sand, stone textures). Low Frequency information is the broad sweep of color and tone (think sky, soft clouds, marshmallows).

Separation means splitting the image so that the color and tone are on one layer and all the texture detail is on another.
If you’re thinking, “Hey, so you just blur the image to nuke the high frequency data,” you’re right. (If you’re not thinking that, reread the last two paragraphs and come back.)
Second things second: why should you care?

Here is an example.

©iStockphoto/ozgurdonmaz
As pretty as the model is, there’s an area in the hair where the color is growing out, and the lighting has exaggerated the imperfect skin texture in several places and created a nasty mottled appearance on her cheek and temple.
In situations like this, you’d normally be in for a lot of meticulous cloning, copious use of the Spot Healing Brush, some dodging and burning, and probably a considerable number of layers, depending on how you normally work.
We’re going to use Frequency Separation to make the work a lot faster, easier and more natural looking than you can achieve with those tools alone.
Setting things up

(Hot Tip: Record these steps as an Action, so you’ll be able to do them with one click every time.)
Step 1: Copy the image layer twice, name the first copy “Low Frequency” (or “LF”, if you prefer) and the second “High Frequency” or “HF.” Then group these two and call the group “Frequency Separation” so that you can mask or modify the blending of both layers at once if you need to.
Hide the High Frequency layer for now.

Step 2: Blur the Low Frequency layer just enough to hide the fine details. How much blur depends on the size of your image and the kind of texture. Gaussian Blur, which I’ve used here, usually works best. After you’ve experimented a bit, you’ll develop an eye for how much is too much.

Step 3: Show the High Frequency layer, select it, and choose “Apply Image…” from the Image menu. The settings to use are

  • Layer: “Low Frequency”,
  • Blending: “Subtract” (to subtract the blurred image from the original),
  • Scale: 2, and
  • Offset: 128 (which sets the basic layer “color” to 50% grey).


That gives you something that looks very much like, but isn’t, a High Pass filter result.

Step 4: Change the Blend Mode to “Linear Light.”
You now have two layers which when combined look identical to the original image.

You’re now set up and ready to begin retouching.
Retouching the uneven tone and color

I’ll start by making that whole cheek area less mottled-looking. The same steps apply for any area where uneven tone and color are a problem.
Step 1: Use the Lasso tool to select the area, switch to Quick Mask mode (keyboard shortcut: Q) and apply enough Gaussian Blur to feather the selection so the changes you make will blend well. Remember the number, because you’re going to use it shortly.

Step 2: Target the Low Frequency layer and apply a large Gaussian Blur to the selected area. Watch the preview and adjust the value. Stop when you have an even texture. Again, don’t overdo it.

Notice that the fine texture of the skin is unaltered. Only the shading and color have been evened out.
Rinse and Repeat: Select the next area (I picked the one between the near eyebrow and the hair), but this time choose Select > Feather Selection (Shift-F6) and enter the feather value from Step 1. You can also set this value in the “Feather” field on the Control Panel, but don’t forget to change it back when you’re done.
Use Ctl-Alt-F/Cmd-Opt-F to bring up Gaussian Blur, adjust if you need to and apply. Select the next area, feather, blur and keep going. It gets really fast.

What about where the hair color is growing out? Switch to the Clone Stamp, set a low opacity value of, say, 30%, and be sure to change the Sample setting to “Current Layer.” This is one time you don’t want to sample all layers!
Clone color from other parts of the hair in Normal mode. You won’t affect the hair detail, because it’s on a different layer; all you’re cloning is tonal value and color.

Retouching texture

To fix texture problems, target the High Frequency layer and use our usual Clone and Healing tools. Make sure that any tool you use is set to sample only the current layer, to save wear and tear on the Undo keys.
In our example, the nose and eye arch need help. Here’s the magical thing about the High Frequency layer: you can clone texture from anywhere. I doesn’t matter what color it is or whether it’s darker or lighter. All you’re copying is fine texture.

There would be more to do on this image, but here’s where we’ve taken a problem image in a very short time, armed with nothing but what Photoshop already provides.

Summary

The Low Frequency Layer is where you adjust color and tone, soften or deepen shadows. All your usual tools work here, but Blur, the Clone Stamp and Dodge/Burn are most used.
The High Frequency Layer is where you fix texture problems, wrinkles and crow’s feet, mostly with the healing tools and Clone Stamp.
There are a number of good video tutorials out there, including this one from Phlearn, who also have made an Action available for download.
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Alan Gilbertson is an independent designer based in Los Angeles. He works across the entire Creative Suite. He can be found answering design and technique questions for designers on graphicdesign.stackexchange.com. His personal blog is at blog.gngcreative.com.

 



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