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December 5, 2013

Advanced sharpening techniques #2 ; High Pass Filter

Photoshop’s sharpening options are varied and powerful, but some more powerful than others, and one tool whose simplicity belies it's power is the High Pass filter, oft overlooked in favour of more traditional techniques like the Unsharp Mask.

The High Pass technique is both highly effective, non-destructive and much simpler to use than the feature rich sharpening filters like the USM filter, so I'm not sure why it goes overlooked, here's the benefits;

1. Fewer steps

2. Affects on the edges, where sharpening is most needed

3. Ignores expanses of even tones and colors, where sharpening isn't desired

4. Noise is not sharpened

Focus on the Edge

The strength of the High Pass filter is it targets exclusively the edges. The rest of the image is left untouched (or unsharpened). But wait! I want to sharpen everything I hear you cry! No you don't.

Sharpening, whichever route you take, is a proccess of increasing the edge contrast to varying levels. The greater the contrast our eyes register between elements in an image, the more our eyes percieve detail and clarity -- "sharpness"

So targeting the edges is exactly what you want and where the High Pass sharpen differs as is it isolates only the edges, applying a 50% grey (neutral) to the rest of the image, which is ignored by the filter -- very handy for not destroying colour, tone, or increasing noise.

High Pass sharpening is more focussed to the areas that need it, and, it reduces the prospect of haloes and artefacts  which we want to avoid at all costs.

Non-Destructive

Aside from being more targeted, the High Pass can be reversed even after the filters applied -- unlike other options. This is because sharpening is done to a separate layer, not to the original source layer. The layer based approach makes comparison of pre and after sharpening as simple as toggling a layer on and off.

Duplicate your layer

With a flattened file ready, make a duplicate layer.

duplicate layer high pass sharpening

All the 'action' will happen on this copy layer, so nothing irrevesrable happens to the image's source data, for me a key advantage of the High Pass technique, it leaves you a untouched original, which makes reviewing 'before and after' easier.

Filter On

With the duplicate layer selected, set your blend mode to Overlay.

high pass sharpening overlay blend selected


Don't panic at the resulting hyper-colour you'll see on your image, that will remove itself during the next step. The reason for setting the blend mode before the sharpen is so can see the sharpening applied in real-time which makes it quicker for you to be able to see what the settings are doing to the image.

Blend Overlay

Now we need to apply the High Pass filter.

Go up to the Filter menu > choose Other > choose High Pass. This brings up the High Pass filter dialog box:

high pass sharpening filter selection

The dialogue box will pop up over your image and at the same time your image will loose it's dayglo look and return to normal. Now you can begin to sharpen the image (and see it's effects in the background).

Compared to the Unsharp Mask Filter options, this High Pass filter is a simplicity embodied, with a single option.

As you drag towards the right, you’ll add more sharpening, and as you drag to the left, you’ll be reducing it. Simple. You’re going to want to start off with a very low radius value, somewhere between 0.5 to 1 -- I.5. I rarely have cause to use anything higher than 2 to be honest -- if the image was captured sharp in camera, which of course it shoud. 5 pixels plus and telltale haloes start to creep in.

high pass sharpening filter options
 

If you can't find a 'perfect' value, err on the side of the oversharpen, as their are several ways to refine the sharpening if needed once you hit OK. If the sharpening needs decreasing more subtly than you can achieve with the radius, here's a few methods to tone it down.

‚Äč1. Opacity

Back in your the layer pallette, with the now sharpenend copy layer selected, reduce the layer opacity. As the filter is applied to the duplicate layer at the top of the stack, we can decrease the opacity by bringing more of the softer source image through.

2. Blend Modes

Overlay blend mode is sort of the “average” amount of sharpening. To increase the sharpening amount, go back up to the Blend Mode options in the top left corner of the Layers palette, click on the down-pointing arrow to the right of the word “Overlay” and change the blend mode to Hard Light, to reduce it try Soft Light.

3. Masks

You can always add a mask to the 'high pass' layer and paint out the elements that are too aggressively sharpened using various opacity with a brush. This is somewhat similar to the Selective Sharpening technique, which you can read more about here.

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