Sharpening a Noise-Reduced Image in Photoshop

Regardless of which sharpening tool you use (Unsharp Mask or Smart Sharpen), be wary of the sharpening process returning some of the noise. If any noise remains in your image—and there usually is some—you will want to carefully monitor any applied sharpening to make sure you do not overdo it. Typically apply only light to moderate sharpening, and perhaps in multiple events with smaller amounts when following a noise/pattern reduction event. This thought process requires that you balance your knowledge—that you typically should oversharpen a bit for printing (to mitigate the slight softening that can occur during the printing process)—and avoid returning any unwanted noise patterns that could show up (or even become enhanced) during printing. Here you will continue work on our noise-reduced beach image:

1. Open your image.
2. Zoom in on an area of the image that shows both the sky background and some highcontrast edges such as the palm fronds and the tree trunks.
3. Choose Filter --> Sharpen --> Unsharp Mask. The Unsharp Mask dialog box appears. You will note that there are three variables to control:


• Amount % This controls how much sharpening will be applied in percentages. A100% Amount will cause a 100% increase in the contrast difference between any two adjacent pixels that are sharpened. So, if there is a 10% difference in the grayscale value between two adjacent pixels prior to sharpening, there will be a 20% difference after 100% sharpening is applied.
• Radius This value determines the width, in pixels, over which the amount of sharpening is applied. Larger values result in thicker and more-obvious sharpened edges.
• Threshold This is the value, in tonal levels, that you use to control which areas of the image will be sharpened. This value dictates the levels of difference in grayscale that must exist between two adjacent pixels before sharpening will be applied. Higher Threshold values restrict the application of sharpening to higher-contrast portions, usually edges, of an image. Threshold is the tool you use to protect low-contrast areas, such as skin tones, from too much sharpening. Threshold is also the key variable you will use to mitigate the return of noise to low-contrast areas when you apply sharpening to images where you have reduced noise.


4. To begin, assign these values, and your image will look like : Amount 200 Radius 2 Threshold 0
   Note two important changes in this image:
   All of the high-contrast edge objects, such as the palm fronds in this example, have harder and higher-contrast edges. In fact, they have very light/dark edges—too much so in fact. The light side of these high-contrast edges is so bright that they form halos. These halos are especially evident when zooming in as shown here, but they will also be obvious at 100% view—too much sharpening. You will want to avoid these kinds of halos.
   - Look carefully at the background sky. Toggle the Preview check box off and on. You can clearly see that the sky now has a mottled appearance due to the sharpening.
5. Keep the Unsharp Mask dialog box open and reassign these values: Amount 50 Radius 1 (a good standard value for 200–300ppi images) Threshold 3. Now you see a more reasonable sharpened image. Note that the high-contrast edges are still harder/sharper than the original image, but are now not sporting those obvious halos. Just as important, setting the Threshold value to 3 has protected the low-contrast background sky from the mottling that was so apparent when the Threshold was 0. You might even decide that you want to apply less sharpening, maybe 25%, if you think the edges are still too abrupt. You will notice that images (like this one) that have been though the mill cannot handle having nearly as much sharpening applied to them—so be  especially careful with images like these to not overdo the sharpening.

Multi-Event Sharpening
Every image presents slightly different sharpening demands. Some images can withstand large amounts of sharpening and look terrific, whereas others, such as noise-reduced images, need to be treated more carefully. A good habit to adopt in these circumstances is to apply sharpening in 50% increments in order to monitor the incremental results. You may even want to switch to 25% increments on the second or third sharpening events. In this manner, you can maximize the amount of sharpening necessary to enhance the focus in your final print, while dodging the return of the unwanted pattern.