Over the last few months we have examined the Unsharp Mask filter in some detail. This filter looks for edges and, if it finds one, enhances it by making the light side lighter and the dark side darker. It creates a halo around the edge and this makes the image look sharper and more convincing, as long as the eye doesn’t perceive it as a halo. There are good images with no sharp edges, and we are going to look at one just now, but most photographs look unconvincing if the eye cannot latch onto something sharp.

Quite often an image still lacks something. We don’t know what it is – the edges have been sharpened, the overall contrast adjusted with Levels and Curves. The whole spectrum between pitch black and pure white is utilised, but the image still looks flat; it doesn’t pop off the paper.

One solution is to apply an S-Curve to the image. This will compress the dynamic range in both the highlights and the shadows and assign a greater percentage of the available dynamic range to the midtones. The eye is more sensitive to midtones and you will end up with an image containing plenty of midtone contrast, but often with too little tonal detail in the shadows and highlights.

If an S-Curve doesn’t help, then it is time for some local contrast enhancement. Whereas the Unsharp Mask filter enhances small-scale edges, local contrast enhancement looks after the large-scale light-dark transitions. We adjust the overall contrast with Levels and Curves, whereas local contrast enhancement boosts the contrast of smaller elements within the image.

We have come across localised adjustments before; you might remember the Local Operators we used for Tone Mapping in HDR images. Photoshop’s Shadow/Highlight tool is another example.

The secret of local contrast enhancement is that it increases ‘local’ contrast in smaller regions, while at the same time preventing an increase in ‘global’ contrast. This is done by making some pixels in the histogram cross over each other, something which cannot be realised with Levels or Curves.

HiraLoam

It might surprise you that we can achieve this effect with our old friend, the Unsharp Mask filter. All we need to do is push the radius to a very high value and compensate with a low amount (high radius – low amount, or HiraLoam). Try it on a number of images and you will discover that most photos benefit from some boost in local contrast, especially if they are taken under dull light, there is haze or lens flare.

My example with the seagull doesn’t really need a boost in local contrast, but I chose that shot to show you how any ordinary image will benefit. Seagull Image 2 A: Original B: HiraLoam C: High Pass sharpening

The settings for the Unsharp Mask filter were: Amount 20%, Radius 60, with a Threshold of 0. A setting of 20% is usually a good starting point, but you can try lower values, down to 5%. The radius affects the size of the transitions, so a smaller radius enhances smaller-scale detail. The setting can vary between 30 and 100 pixels. The Threshold sets the minimum brightness change that will be enhanced. This is rarely used in local contrast enhancement and usually stays at 0.

The radius setting is very dependent on the image size (resolution), even more so than with normal sharpening. High resolution images require a high radius setting. Very small images may need a radius setting of even less than 30 pixels.

As with sharpening, local contrast enhancement can introduce unwanted colour changes. It can also increase colour saturation significantly. You can run the Filter on the lightness channel in LAB or just run it in RGB and then use Edit > Fade in Luminosity mode, as we did for sharpening.

In general, this method should be used with care. It can detract from the ‘smoothness’ of tones within your image, thereby changing its mood. Portrait photography in particular can suffer when overdoing this technique.

The Clarity Control in ACR

If you work with Raw files, then local contrast enhancement won’t be anything new to you. The Clarity slider on the ‘Basic’ tab was introduced in ACR 4, which came with Photoshop CS3. This slider does the same as HiraLoam. Of course, Lightroom has also got it and don’t forget ACR works on TIFF and JPEG files as well. Photoshop’s Unsharp Mask filter with its three sliders gives you more control, but most times the job can be done during Raw conversion. You will find that almost every image can do with at least a small dose of Clarity.

If you have the latest ACR 5 or Lightroom 2, you might have noticed that the Clarity slider goes all the way down to -100. This means that we can reduce the local contrast, something we can’t do with the Unsharp Mask filter.

The first application for a negative clarity is for portrait work. Skin blemishes, pores and small wrinkles start to vanish, to be replaced by a smooth complexion. It is similar to the soft focus effect that we examined some time ago (TPM October 2006).

You can make a universal adjustment by simply moving the clarity slider into negative territory. Another, more targeted way is to use the new Adjustment Brush only on the skin areas. Hit ‘K’ to open the Adjustment Brush, set the Clarity slider to about -50 and brush over the skin. Then fine-tune the Clarity setting to achieve the look you want. You can then click on ‘New’ and use a positive setting and brush over hair, eyes and mouth to add some snap to those areas.

Another application for negative Clarity and Adjustment Brush is for isolating your subject by softening the background. It is not as versatile as the Lens Blur filter in Photoshop, but quite handy for a quick blur of unwanted detail – and it is non-destructive! This method is also great for softening any light source that is in the picture.

Brush over the light bulbs or even better, over lit candles to create a dreamy mood. A third application, which I want to demonstrate with the wedding shot, is to create a soft glow around a subject. Also note the very different looking background.

Image 3: Wedding. Left: Clarity +30; Right: Clarity -100

High Pass Sharpening

There is one more technique which is normally associated with sharpening, but more often than not relies on local contrast enhancement.

You start by making a copy of the image layer. Change the blending mode to Overlay. Overlay blending is essentially a contrast multiplier and you will get a massive (and typically ugly) increase in contrast. Next, you need to apply the High Pass filter (under Filter > Other). The Radius setting depends on the image resolution. Used as a sharpening tool, the radius is set to between 0.8 and 3 pixels. For local contrast enhancement use a radius setting of around 10, but that can be increased, depending on the effect you are trying to achieve.

You can see the High Pass sharpening (radius 10) on the seagull in Image 2 (C) and compare it to the HiraLoam treatment. High Pass sharpening adds quite a strong halo effect, if the image contains sharp edges. I normally reserve High Pass sharpening for images with no sharp edges which don’t take to normal sharpening with the Unsharp Mask filter. Think of a portrait taken with a special soft lens.

Another example which I use here is one of Andris Apse’s superb shots (below). The USM filter, even when used with an edge mask, would only bring out image noise or film grain.

Image 4 & 5: Landscape by Andris Apse. Top: before High Pass sharpening Bottom: after High Pass Sharpening

If the effect is too strong you can reduce the opacity of the top layer. The sharpening effect can also be drastically increased by using the ‘Hard Light’ blending mode instead of ‘Overlay’. The ‘Soft Light’ blending mode will give you a more subdued effect

Many thanks to Andris Apse for allowing me to use one of his images.

Words: Hans Weichselbaum

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