Adobe Photoshop, for example, does not provide such a simple way to modify a picture local contrast and by its own nature, this kind of adjustment cannot be achieved with tools that modify the overall image, no matter how complex they are: you are not going to raise only the local contrast using the Contrast, the Levels or the Curves adjustment.
Even if you're a Lightroom user, there are times when you're going to use Photoshop (or similar programs) in your workflow: I deem necessary to know how you can achieve the same result when you cannot rely either on Camera RAW or Lightroom. The good news is that you can achieve the result both in Photoshop and in Photoshop Elements, the stripped down version of Photoshop aimed at beginners and amateurs.
The Unsharp Mask
The Unsharp mask is a tool that's best known to Photoshop users to sharpen an image. It's funny how names can be so misleading at times, isn't it? If you're interested about the history of the technique this name comes from, you can have a look at the Wikipedia article about it.However, as far as we're concerned, the important thing is the following: the unsharp mask can be used to raise the local contrast of an image in a very simple way.
You can see the Unsharp mask window in Photoshop Express in the following picture:
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| Photoshop Elements - Unshard Mask Window |
Unsharp mask adjustments are applied according on three parameters:
- Amount.
- Radius.
- Threshold.
Usually, when you use the Unsharp mask tool to sharpen an image you select a relatively small radius and a large amount. On the other hand, if you use a relatively large radius and a small amount, the effect will be a local contrast enhancement. If you read the Wikipedia article about this technique, the radius is used to tell how much the image should be blurred during the process of detecting transitions.
In the following example, you can see the effect on the same image we used as an example of the Clarity adjustment in the aforementioned post:
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| Original Image |
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| Unsharp Mask - Amount 20%, Radius 50, Threshold 0 |
What About Negative Clarity (a.k.a., Reducing Local Contrast)?
The Unsharp mask is an easy way to enhance the local contrast of an image: so far so good. But if you've used the Lightroom Clarity tool, you're probably missing negative clarity adjustments as well! There's no way to reduce the local contrast using the technique seen so far and you must rely on other tools.To understand how you can achieve negative clarity, we will first look at how positive clarity can be achieved manually (reproducing the steps that are performed by the Unsharp mask tool).
The first steps is detecting transitions. In film photography, the process was pretty complex. Photoshop makes it easy: that's what the High pass filter is meant for. If we use a radius of 1.5 pixels, this is the result:
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| High Pass Filter - Radius 1.5 pixels |
The result is:
- Neutral grey pixels where no transitions were detected.
- Colored pixels where transitions were detected.
This new layer will be superimposed to the original using the Overlay blending mode. The neutral grey pixels won't affect the underlaying pixels while pixels marking transactions will enhance the local contrast. This happens because the Overlay blending mode multiplies (darkens) dark areas and screens (lightens) light ones. The final result is the following:
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| High Pass Filtered Layer Overlaid on the Original Image |
There's no doubt that using the Unsharp mask tool is much quicker when we need to enhance local contrast. What about negative clarity, then? Simply invert the layer generated by the high pass filter:
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| High Pass Filtered Layer Inverted and Overlaid on the Original Image |
Pretty much the same effect you'd get in Lightroom using negative clarity, only a bit more difficult.
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