Graphics: The clearer picture: Part II

It's a sad fact that blurring some­times will creep into photo­graphs which cannot be retaken. The problem can be knocked back with a graphics editor but, unfortunately, the more drastic blurs can't be fixed - and some repairs may introduce unwanted effects. As always, it's best to avoid the problem when taking the picture rather than deal with the consequences. See last month's story.

Most types of blur will smear the colours of a picture. Sharp lines become fuzzy and colours saunter into places where they don't belong. This makes blurring difficult to fix: it's a bit like try to unmix two different colours of paint. The greater the blur, the more the colours mix and the harder your task.

If you hunt around the Web for ways of correcting blur, beware of miracle tools. Most editing programs such as Photoshop and Paint Shop Pro can achieve similar results without the need to fork out extra moolah. Claims about such programs are frequently extravagant and details are thin, but most seem to use the same basic techniques of sharpening available in your graphics programs. Which brings us to the first problem: definitions.

Despite the terminology used in many graphics programs, 'sharpening' an image is not the same as improving focus, nor will it magically undo severe motion blur or camera shake. Here's why:

Sharpen tools increase the contrast of adjacent pixels, giving the appearance of stronger lines. The result may appear sharper, but the image frequently appears speckled. Think of sharpening like taking a fine pen and drawing around edges of large dots to make them more defined. As you can probably guess, this technique strengthens the edge lines but it doesn't put all the smeared lines back into their correct positions. If a sharpening tool is used too harshly, it can cause colours to bunch together, creating blotches and unnatural-looking colours.

If the image is hopelessly blurred or out of focus, a graphics program is not going to undo the damage. However, in desperate situations, you can always give it a try. If the problem is slight, you can fix it.

Start by determining the location of the blur - is it the entire picture or only certain parts? For images in which a few elements are affected, you'll probably need to isolate that area by using a selection tool (take a look at the April/Junes issues). Use feathering options to help the boundaries mix together. Apply any sharpening tricks only to the blurred area rather than the entire picture. If you sharpen everything, it may make the blurred features more obvious or introduce new problems.

In your graphics program, hunt around for the Unsharp Mask filter (usually located in the Filters Menu under Sharpen). Skip the other filter types such as Sharpen, which are not well suited to photo­graphs. The Unsharp Mask has three settings, each controlling different aspects:

Strength: controls the contrast change along the edges in your photo, making dark colours darker and light colours lighter. As a general rule, 100 to 150 per cent is the most useful range for this variable. Start with 100 and see if you like the effect.

Radius: specifies how far from an edge - measured in pixels - the effect should go. Set your radius to 1 or 2 for a small image and 3-5 for larger images.

Clipping: also called threshold, it controls how much contrast must already be in the image before the tool will apply any sharpening. The lower the number you enter, the more sharpening you'll get.

The best approach

Due to the way the Unsharp mask works, it's most successful when dealing with black and white elements or images. There's a groovy little trick that can let you treat your image like it's black and white, without the need to remove the colour: the Lab colour mode.

Unfortunately, it's not a very common feature, but it is included in programs such as Photoshop. Select Image-Mode-Lab Color. Now switch to the Lightness channel (<Ctrl>-L). Your onscreen image should look black and white. Select Filters-Sharpen-Unsharp Mask... Adjust the three sliders until the edges look crisp, but don't go overboard. When you are done, hit <Ctrl>-~ (yes, that's a tilde) to select all the channels again.

Final tip: switch back to RGB or CMYK when you are finished; otherwise, programs that don't support Lab mode won't be able to open the file.

The yawning tiger image suffers from slight blurring - just enough to ruin the picture. By zooming in on the tiger's nose, it's possible to see the impact of different sharpening techniques. The first image is the original and its smeared edges are apparent. By switching to Lightness in Lab colour mode, the middle picture is showing much better definition. The third picture has the same Unsharp Mask settings as the middle image, but they were applied to the entire picture. Note the splotches of colour in the last image - particularly purple and yellow - which create the speckled look common with the Unsharp mask.

Dave Johnson contributed to this article.

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