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Sharpen Up Your Image Don Bradbury looks at digital image sharpening tools Digital cameras offer the user many more creativity options in comparison with conventional photography. In fact the range of things you can do with an image in digital form is virtually endless. Take, as just one example, the sharpness of an image. Most of us have had the odd ‘less than biting’ image from our digicam, and, where the subject is suitable, we might lean upon our photo editing software, whatever it might be, to lend assistance in that direction. Let’s use the amateur’s favourite, Paint Shop Pro, to see how the job it done. But whatever you use - Photoshop or what have you - the process is essentially the same. Load the image into the editing program, carefully crop it to the desired shape, resize it if you have to - for emailing perhaps - and then select from the menus Image/Sharpen (or the equivalent). Sharpening options Depending on the software you use, you might see sub-options labelled Sharpen, Sharpen More, and Unsharp Mask. These are all image sharpening tools, though the last achieves the objective rather differently, and in a more subtle manner compared with the former two. We’ll come to that in a minute. Start with Sharpen and try each of them on your chosen image now using the default settings in the case of Unsharp Mask.
Don’t worry, provided you don’t save the changes over the top of your original file such changes can be undone by selecting Edit/Undo (or equivalent). Do that after each test before you run the next. Notice the progressive difference? The eagle-eyed, and even those with aging eyes, can hardly fail to notice some effect, though it’s unlikely to be great with the basic Sharpen tool. However, some subjects show the sharpening effect more than others, and in any event Sharpen More will always give a more noticeable effect. Unsharp Mask This is a special tool that, rather than simply increasing contrast at every image boundary in sight, as is the case with Sharpen and Sharpen More, detects and increases the contrast at specific subject boundaries - which is probably what you really wanted all along. Unsharp Mask user settings for Radius, Strength, and Clipping will be seen. These determine the boundary detection parameters, the depth of the change (in terms of image intrusion), and the overall effect of the tool, ie the increase in adjacent pixel contrast. Vary each of these parameters to judge the effect, once again bearing in mind that their influence depends on the subject and hence cannot be held as universally preferred settings. That is to say, you will want to change them for different subjects. Taken too far, unpleasant outlining of image boundaries will be seen after using Unsharp Mask. Back off on Radius and Strength settings until you are happy. Judge the effect better my using the zoom tool by all means, but remember that it’s the visibility in the final print that really matters, so a test print or two may be in order. Unsharp Mask in more detail Unsharp Mask locates each pair of adjacent pixels with a difference in brightness that you specify. ‘Clipping’ or ‘Threshold’ values between 2 and 20 should be tried, and bear in mind that a threshold value of zero sharpens all the pixels in the image (like Sharpen). It then increases the pixels’ contrast, again by an amount you specify (Strength); 150% and 200% is generally sufficient. Further, you specify the number of surrounding pixels to which the sharpening effect is applied; such a ‘Radius’ between 1 and 2 is usually appropriate. Remember, the effect of Unsharp Mask is more pronounced on-screen than in high-resolution output, so some experimentation is called for. Beware that delicate flesh tones, in particular, are not the place to introduce excessive noise into the image - which Unsharp Mask can do when applied extravagantly. So take care, but do give your sharpening tools a try.
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