By Crawford Hart.

Part 1 of this series provided an overview of some of the generally accepted approaches to the problem of achieving perfect skin. The techniques themselves aren't glamorous, though the results can be. They tend, rather, to be boring, repetitive, simple procedures, tasks in which the operator must do most of the work. Grunt work, in other words.

Part 2 (the one you're reading now) and Part 3 (to come) will shift over to those effects in which Photoshop does the work and we just add up numbers and push buttons, though a little bit of delving into the esoterica of Photoshop, how it works under the hood, will be called for. But the payoff is definitely worth it. The ideas behind the process discussed here certainly did not originate with me; they can be found on other forums all over the web. I've provided my own particular slant, as I would expect you to do, once you're comfortable with the basic ideas.

What we're going to do is take this image, generously donated for this purpose by ricbuchner, from its original state...

...to here, without lifting a clone tool or healing brush, without dodging or burning or patching or anything else. Now, it must be said, the image we are starting with comes in pretty good condition, both in terms of the photography, and in terms of the skin quality. No scars, ridges, bumps or gross discolorations. So, of course, we're going to degrade things a bit before embarking on the restoration.

This shows the result of some channel blends that I've been playing around with, splitting up the R, G, and B channels and blending them back into the image in various modes to create more dramatic contrast. These blends and their possibilities will be covered in Part III, but, as you can see, including the blue plate to add weight also brings out imperfections that were mostly yellow discolorations in the original. An ugly mess. if we could just get rid of the blotches, the gain in contrast would definitely be worth it. It's out of the question to try to dodge and burn them away, and the healing brush would just stir up the soup. In fact, it's such a disaster that the idea of even trying such a move probably would never come up in a normal work flow. Better to just rely on curves for contrast, though I've found that they don't do the job quite as well.

And of course, blurring is a rigid taboo. Any blur that could get rid of the blotches would certainly lose any useful detail. Wouldn't it? Turns out, the answer is "Not necessarily."

THE BASIC IDEA
The theory sound simple enough: blur the big stuff and leave the small stuff alone. We want to lose the blotches but keep the pores and other fine details that make skin texture believable, and, the lack of which, conjours up the dreaded judgment "AIRBRUSHED!!!"

But if it sounds simple, it also sounds impossible, until we actually explore the relationship between blurring and sharpening in Photoshop, specifically between the Gaussian Blur filter and the High-Pass filter.

Rather than get into an elaborate discussion that is ultimately beside the point, try this yourself with any image you wish. Or you can just watch me as I play around.

NOTE: If you chose to pull the example images posted here, your numbers will be different than mine. I worked with an original Hi-res image; these are scaled down for the purposes of illustrating the steps.)

BLURRING AND SHARPENING
• Start by duplicating an image twice into new layers. Leave the original alone. We'll use it for comparison.

• To the first copy apply a Gaussian Blur such as you see here. The radius will vary, depending on the image size and resolution. The goal is a broad blur that wipes out all fine detail, leaving only areas of color. In this case I used a radius of 25, but the numbers aren't important. Getting rid of the details is the point.

While the Gaussian Blur filter is well-known, the High Pass Filter is a little stranger, less user friendly and not so easily understood. It is often used for various sharpening moves. Depending on the radius, it reveals fine detail against a 50% gray background. The larger the radius, the more detail is exposed.

When used as a sharpening tool, the idea has been to combine the resulting layer with the underlying image in one of the modes that both lighten and darken, based on the value of the pixels on the top layer. The usual modes are Soft Light, Overlay and Hard Light, modes for which 50% gray has no effect and so only work on the level of detail revealed.

Go now to the top layer in your file and choose Filters>Other>High Pass.

Like Gaussian Blur, there is only one value to set, and it too is a radius. Use the same value that you used for your Gaussian Blur layer. It will look more or less like this.

Note how each layer contains precisely the information that the other is lacking. It might have been more helpful had Adobe extended the Audio metaphor of the High-Pass filter and called the Gaussian Blur filter a Low Pass filter.

Note how effectively the High-Pass layer can restore detail to the blurred layer underneath, when used in the modes previously mentioned. All of these are at 100% opacity. If you turn off the blurred layer, so that the High-pass layer is blending into the original image, you will see how the filter can be used as a sharpening method. It's effective, but not nearly as effective as it will be once you understand what is really happening and can exploit the effect to its fullest potential.

TWO SIDES OF THE SAME COIN
Turn the blurred layer back on

Set the blending mode of the high pass layer to Linear Light, and the opacity to 50%. (The math isn't relevant to using the modes correctly; it's enough to know that Linear Light has a much stronger blending effect than any of the other modes. It is the same as using Additive in Calculations or Apply Image.)

With the exception of the very lightest highlights, which have gotten lost in translation, we have not only restored detail, we have virtually returned the image back to its original state. Toggling back and forth from the bottom layer alone to the two layers on top will quickly confirm this.

You can experiment with the opacity of the High Pass Layer: raising it above 50% sharpens the image, lowering it softens, a nifty trick in itself. But not the one we're ultimately interested in.

What this demonstrates is that High-Pass and Gaussian Blur are simply two sides of the same operation. Gaussian Blur is the low pass filter, blocking out all the fine detail and only allowing the largest, most basic areas through. The level of detail is based on the radius setting.

The High-Pass filter works the same way, but in reverse. It blocks the lower bands of detail and only allows the higher bands of information, the fine detail, to pass through.

The radius is the key. Different kinds of detail are maximized at different radius settings. The trick will be to isolate those different "frequencies" of detail, emphasize the ones we want, and minimize the stuff we want to get rid of.