A common misunderstanding about image sharpening is that it can save a blurry or out-of-focus image. It can’t. Sharpening tools can do a lot to enhance your pictures, but don’t try to use it to fix an out-of-focus original.
That said, all images shot digitally, regardless of how well you capture them, usually require some sharpening to match what the lens saw. This is because as light passes through the lens to the sensor and data is converted from analog to digital, some softening occurs. If you’re shooting in JPEG format, sharpening can take place in the camera’s internal-processing software, but additional global and selective sharpening is usually needed, too. If you shoot RAW files, you’ll definitely need to do some sharpening in postprocessing.
What Sharpening Does
Sharpening increases the contrast between light and dark pixels at the edges of objects in your image. In sharp photos, the transition from light to dark happens quickly, over a small area. In soft images,the transition is blurred across a larger area. So by enhancing the difference in tone at the edges of objects in a scene, the image appears sharper.
Keep in mind that you don’t have to sharpen the entire image uniformly. In fact, it’s often better to make this adjustment selectively.
The most practical way to sharpen is simply by adjusting the sharply focused parts of a photo and leaving the rest alone. Depending on the scene, each image requires a different degree of sharpening.
For example, a landscape with fine details will handle more sharpening than a portrait where you want to keep skin tones smooth. So sharpening selectively is often the way to go. If your image has areas that are out of focus, overall sharpening can cause problems because it affects tonal details, and blurry areas tend to look best when left unaltered.
A popular sharpening tool is Unsharp Mask in Photoshop because it allows a great degree of control. It’s found in the Filter menu (Filter > Sharpen > Unsharp Mask). This tool has three sliders. Amount is the top slider, which determines the intensity of the effect and ranges from 1% to 500%. Image resolution influences this setting, with higher-resolution cameras allowing for values as high as 200% or more.
The middle slider, Radius, controls the width of the edge sharpening and ranges from 0.1 to 250 pixels. Typically, the Radius is small, ranging from 0.3 to 2.0 pixels. If you see a halo effect around the edges of objects, the Radius is too big. Larger image files can handle higher Radius settings better than smaller files.
Finally, the Threshold slider determines the pixels to be sharpened based on the difference in brightness between neighboring pixels. For example, a Threshold of 3 affects all pixels that have tonal values that differ by a value of 3 or more, on a scale from 0 to 255. So, if adjacent pixels have tonal values of 100 and 102, they’re not affected by sharpening.
For fine details, a higher Amount and lower Threshold work best, while portraits and areas with smooth tones work better with a lower Amount and a higher Threshold.
Each image requires a different degree of sharpening, but here are some guiding points: With portraits, start off with an Amount of 100, a Radius of 1.0 and a Threshold of 6. For buildings or landscapes, try an Amount of 150, a Radius of 1.5 and a Threshold of 3. If there’s a lot of noise in an image, try going up to a Threshold of 10 or 12. Remember, experimentation is key.
Another tool within Photoshop is the Smart Sharpen filter, which allows you to set the sharpening algorithm or control the amount of sharpening that occurs in shadow and highlight areas. One of three sharpening algorithms can be used: Remove Gaussian Blur, Lens Blur or Motion Blur. One drawback to this tool is that it doesn’t have a Threshold setting so it can overemphasize noise.
For Aperture users, the Sharpen and Edge Sharpen tools are good to know. Sharpen has two sliders for setting intensity and pixel radius, and Edge Sharpen has three sliders for setting intensity, edges and falloff. Intensity controls the amount of sharpening. The Edges slider lets you set the number of pixels that qualify as edges to be sharpened, and the Falloff slider produces a more or less pronounced effect. An easy and more intuitive sharpening control is Nik Software’s Sharpener Pro 3.0. The plug-in provides a straightforward interface that sharpens images based on resolution, image and print size, as well as the expected viewing distance between the subject and the final photograph. U Point-powered Control Points allow you to make precise and selective adjustments to sharpness and detail without having to create layer masks.
Avoid Too Much
If you see halos in your image, you’ve gone too far. Halos are those bright edges that look unnatural. You’ve also sharpened too much if you see a loss in tonal range or the image simply looks too harsh. If you’re conservative in the amount of sharpening you use, you won’t have that negative “digital” look that too much sharpening produces.
Why It’s The Final Step
Usually, it’s best to sharpen last, after you’ve finished making all other adjustments—color correction, saturation, distortion corrections and more. These adjustments can affect the overall contrast of the image and the apparent sharpness. Another reason to sharpen last is that the amount of sharpening you need depends on your ultimate use and output size. Images that you plan to use on the web typically will need less sharpening than those you plan to print.
Ultimately, your eye is the best judge of how much sharpening to apply. If it looks unnatural, you’ve gone too far. Keep a copy of your original file so you can always go back and adjust sharpening for different uses and sizes.