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Articles: Using the unsharp mask filter in Photoshop – 016

Give your images that extra snap. Hans Weichselbaum explains how…

Sharpening is one of the most overlooked yet essential factors of digital imaging. In the first part of this series we had a close look at the primary sharpening tool, the Unsharp Mask filter (USM). This month we examine ways of getting the best out of the USM.

One way is to run the filter not on the whole image but on one or two channels, or only on parts of the image that need sharpening, such as the eyes and the mouth in a portrait. However, the most elegant solution is to create a selection that covers all the contrast edges of the image; only those parts get sharpened.

There is also a way to sharpen an image through local contrast enhancement, either through the High-pass filter or with the USM filter, using a very high radius setting. Finally, we will look at a number of commercial sharpening programs.

SHARPENING INDIVIDUAL CHANNELS

The danger with sharpening is that we can accentuate details we don’t want to highlight, such as noise, dust, grain, colour artefacts or little imperfections on the face of the bride. The third slider (Threshold) of the USM filter gives some control but not enough.

CCDs in cameras and scanners produce noise. The smaller the CCD and the less light it gets (high ISO setting), the more the weak signal needs to be amplified, which leads to noise. Even drum scans can be noisy due to interferences between the pixels and the film grain.

Have a look at the individual channels of an RGB file. The blue channel is almost invariably the noisiest. Similar to our eyes, CCDs are less sensitive to blue light and will add noise, which appears as random pixels, especially in dark shadow areas. Fortunately, the blue channel is usually the one with the least important details. There is little harm in running the Despeckle filter on the blue channel (or use the Dust and Scratch filter for more control). The red and green channels can then be sharpened more aggressively. Use the same Radius setting, but with a higher Amount on green than on red.

Sometimes we find colour artefacts in our images, moire patterns or colour fringing along edges from chromatic aberration. Sharpening will accentuate these colour problems. We can avoid this by converting the image to Lab mode and only sharpening the Lightness channel.

Changing the colour mode from RGB to Lab and back will degrade the image, but we can get around this by using the following technique: apply the USM filter to your RGB image, then select Edit > Fade from the menu bar. Change the blending mode from Normal to Luminosity and click OK. This will have the same effect as sharpening the Lightness channel in Lab, but is quicker and will avoid quantisation errors when changing colour modes.
However, in more severe cases it is best to convert to Lab, sharpen the ˜L’ channel and apply some blurring to the ˜a’ and ˜b’ colour channels. This can be done with the Median filter, the Dust and Scratch, or the Gaussian Blur filter. I prefer the Dust and Scratch filter ” the Unsharp Mask of blurring ” because it gives better control.

SUPPRESSING NOISE WITH MASKS

If noise problems are confined to the shadow areas, it makes sense to mask them out before sharpening. The important details are usually in the midtones and highlights anyway.

A quick way to do that is to go to ˜Select’ in the menu bar, click on ˜Colour range’ and choose ˜Shadows’ in the drop-down menu. Tick the ˜Invert’ box and click OK. We now have midtones and highlights selected. You can expand or contract the selection, or do some local touch-up with the paintbrush in Quick Mask mode. To avoid abrupt transitions, the selection needs to be feathered by a few pixels, depending on image size. Then we are ready to apply a full-strength sharpening to the more important areas of the image.

Similarly, we can protect large sky areas, which are also quite often noisy.

CAMERA RAW VS JPEG

Today most images come from digital cameras. If you set your camera to JPEG, the camera will do the sharpening. In most cases it will be okay, or even good, but seldom perfect.

To get the best quality, you need to switch to RAW files and do the sharpening either during or after the RAW conversion.

The Adobe Camera RAW converter has a sharpening tool, which you can find on the ˜Detail’ tab. With only one slider, it does not offer a lot of control and I recommend sharpening later in Photoshop.

To better judge the conversion process, there is a way to have a sharp preview without actually applying any sharpening to the file: click on the little triangle at the top right, which opens up the Camera Raw Preferences. Here we have the choice of applying the sharpening to the actual file, or to the preview only.

SMART SHARPENING

The ideal way of sharpening would be to only apply USM to the edges in our image and not exaggerate the texture in flat areas. There is a filter called ˜Find Edges’ in Photoshop. Let’s see what we can do with it.

  • Go to the Channels palette and click on each of the three colour channels. We look for the channel with the highest edge contrast. Duplicate that channel by dragging it to the New Channel button. (Needless to say, we omit step 1 with B&W images).
  • Select Filter > Stylise > Find Edges. This will be the mask through which we apply the USM filter. The darker the area, the more sharpening it will get.
  • Select Filter > Blur > Gaussian Blur. The Blur filter will smooth out the transition between areas that get sharpened and those which don’t. The radius setting will depend on the resolution of the image.
  • Select Image > Adjustments > Levels. Here we can increase the contrast of our mask by moving the black and white sliders towards the middle.
  • Choose Select > Load Selection from the menu. This makes a selection from the modified channel. Check the ˜Invert’ box to select the black areas for sharpening.

Click on the RGB channel to return to the normal view and apply the USM filter. We can use a fairly strong setting now, say 250/1/0 or more, because we are sharpening only the important areas. You can get rid of the ˜marching ants’ by pressing Ctrl+H and observe the outcome in 100% view.

This is my preferred method of sharpening. The whole procedure looks quite daunting and very time consuming, but it can be automated. Performed as an action, it only takes a few seconds. I also recommend running it on a separate layer on top of the original. This allows you to later use the Rubber tool on the top layer to reduce the sharpening effect in areas where you don’t want it or, even better, work with a layer mask.

OTHER SHARPENING TECHNIQUES

Most of my own and my clients’ images get the Smart Sharpening treatment, but there are a few exceptions. In the first part of this series I said most (though not all) good images have a strong edge definition. An example of a perfectly good image, but without sharp edges, would be a portrait taken with a special soft lens. We wouldn’t want to sharpen that.

Another example, shown here, is one of Andris Apse’s superb shots, a scan from a 6x17cm film. The USM filter, even in Smart Sharpening mode, would only bring out the film grain. These images often benefit from some sharpening which does not rely on USM. The most common technique is based on the High-pass filter:

  • Duplicate the background layer
  • Select Filter > Other > High-pass
  • The radius setting can be between 1.5 and 10.
  • Change the blending mode to ˜overlay’, ˜soft light’ or ˜hard light’, depending on the desired effect.
  • Decrease opacity of the layer, if necessary.

A low radius setting is seen as ˜sharpening’. At the high end, around 10, it is more of a ˜local contrast enhancement’. I personally find images that don’t respond well to a USM filter treatment need a radius setting of around 10 in the High-pass filter.

Another interesting way of local contrast enhancement is by running the image through the USM filter, but with a low amount (10-30%) and a high radius setting (20-40).

CONCLUSION

Sharpening can give your images the extra snap that makes them jump off the page. Too little sharpening will leave them looking dull and, worse, too much sharpening gives them the unpleasant ˜crunchy’ look. It is essential to have a good grasp of how the USM filter works, to know what the three controls are doing and how they interact.

The next step is to apply the USM filter selectively. By sharpening individual channels and selected areas, and possibly blurring other parts, we can transform an ordinary photograph into a work of art.

Digital sharpening is definitely one of those skills that develop with practise and experience. Expect your results to improve over time and, since sharpening cannot be undone, this is one good reason to always keep a copy of your unsharpened files.

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

Posted by D-Photo on August 4th, 2007 in Articles

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