Sharpening Your Images

If there's one thing about editing digital photos that frustrates people, it's how to sharpen them. There are two unwritten truths in digital photography:

• Every digital photo needs to be sharpened.

• Everyone oversharpens their images until they learn how to do it correctly.

Luckily, I'm going to teach you how to get past truth number two.

Unsharp Mask Explained

The term Unsharp Mask is often confusing. If you came from a traditional film and darkroom background, you're familiar with the process of masking during development. But for digital photographers, or those who have never spent time in a darkroom, the idea of masking can be hard to grasp. Essentially, in masking you're masking, or hiding, the "unsharp" areas. In digital photography, the job of masking is done by enhancing the contrast between edges in your photo.

Photoshop does this with the Unsharp Mask filter (Filter > Sharpen > Unsharp Mask) (Figure 8.14). Other image-editing programs have similar controls, sometimes with different names, but always with the same method of sharpening. With three controls available to define how edges are found and enhanced, you have a great deal of control over the final result. Let's take a closer look at each of the Unsharp Mask controls to understand how they work.

Figure 8.14. Unsharp Mask is a powerful filter that allows you to improve the appearance of your digital photos. Don't be fooled into thinking it will fix a blurry photo, though.

• Amount controls how strongly the contrast effect is applied to your edges. Essentially, you're increasing the percentage of contrast between pixels to give the appearance of a sharp edge. The sharpening appears in your image as a halo along the edges affected by the filter.

• Radius determines the pixel width of the edge that will be affected by the amount of increased contrast. Radius is the most important adjustment of the three controls and can ruin a photo quicker than you can say, "Oops." Figure 8.15 is an example of what happens when the Radius is set too high. You can see the distinct halo, or glow, around the edges of Devil's Tower. Last time I checked, Steven Spielberg notwithstanding, this wasn't a natural occurrence here. And very few things scream "digital" like an oversharpened image.

  Figure 8.15. Oversharpening a photo adds an unnatural-looking halo around the edges of the image.

  

• Threshold defines what constitutes an edge by determining how much difference in tonal value is required between adjacent pixels for them to be considered an edge. A low setting of 3 is adequate for most images and means that any adjoining pixels with a difference in tonal value of 3 or more will be considered an edge that will have contrast applied to it for sharpening. A higher setting requires the filter to look for a greater difference in tonal values between adjacent pixels and reduces the number of edges, resulting in less overall sharpening. Threshold values range from 0-255, with 0 requiring no tonal difference (all pixels are edges) and sharpening applied to the entire image. With a setting of 255, no pixels qualify as an edge, and the image receives no sharpening at all.

Now, by themselves these controls aren't hard to understand or master. It's the interaction among them that can lead to hair loss and increased stress. You see, changing Radius affects how strongly Amount appears, and adjusting Threshold can magnify or reduce the effect of both Amount and Radius. Luckily, the controls are presented in the general order of use. You will typically make adjustments to Amount first, to get a rough value for how much sharpening to apply to the photo. Then, by adjusting Radius, you control how prominent that sharpening is. Finally, you can fine tune the results with the Threshold control.

The Sharpening Process in Action

Sharpening differs based on the intended outputincluding size and media. It makes sense then that you don't want to apply sharpening to your file until all other editing tasks, including resizing, are completed.

In Figure 8.16, you see an original image with no sharpening applied. Edges are soft, and detail tends to blur together slightly, giving the entire image the appearance of being a bit out of focus.

Figure 8.16. Here's the original image prior to sharpening. Edges are soft with a lack of detail throughout.

The first step in sharpening this image is to set the Radius to a rough estimate of what I think the image will need. In this case, I've selected a width of .8, which is a good compromise number. A pixel width higher than 1.0 in most images can begin to adversely increase the halo, or glow, around the images' edges. Next, I set the Amount to control how strongly the effect is applied (Figure 8.17). In this case, a setting of 180% gives me the look I want without overemphasizing the edges. Finally, I set the Threshold to 1, which prevents lower contrast areas from being affected but still finds the edges of tiles and roof lines to produce the sharpening shown in Figure 8.18.

Figure 8.17. After setting the Amount control, things are looking much better.

Figure 8.18. Here I've fine-tuned the sharpening effect with the Threshold slider.

A Few Sharpening Guidelines

Although every image situation is unique, there are some general guidelines that will help you with using the sharpening tools. I go into these in great detail in my book Print Like a Pro (Peachpit Press, 2006), where I have an entire chapter devoted to the magic of sharpening.

• People are soft. People don't need to look as sharp as other subjects. Lower Radius and Amount settings are better when doing portraits. Particularly with women and children, you normally want to avoid enhancing contrast in the skin. The only key element that you should be going for is sharpness in the eyes. If your software supports it, select the eyes and sharpen them separately.

• High detail, low radius. Highly detailed images work best with low Radius settings in the range of .4 to 1.0, and a higher Amount setting in the 200%275% range. Keeping the Threshold at a low setting of 03 is normally the best choice here.

• Low detail, high radius. Images with low detail, or ones that you want to avoid strong edges in, such as portraits, require higher Radius setting, often in the 24 range. Conversely, you want to lower the Amount to 75150% on average to avoid overemphasizing the edges. Increasing the Threshold setting to a range of 712 is typical for this type of image.