Photoshop – Sharpening

Sharpening (credit to Clint Jukkala, Yale University, 2008 for text)

Digital images, whether from a scanner or a camera need to be sharpened. There are several reasons why. First, a digital image is not continuous in tone. As we know digital photographs are made up of a finite range of tones, around 256 to be exact. The transition of these tones tends to be fuzzy because of the way digital sensors translate light and because sensors often have a blur filter in front of them. This filter prevents fine details from slipping into the tiny spaces between pixels. In rare cases sharpening can enhance areas of a camera image that were not sharp to begin with due to missed focus or camera movement during exposure. Generally, though, sharpening is used to bring back some of the detail that gets lost in digital translation. The tool we will use is called Unsharp Mask. The name of this toll harkens back to the days when printers would sandwich a sharp and a blurry negative of the same image together before printing it. This sandwiching increased edge contrast and gave the illusion of sharpness. Unsharp Mask in Photoshop works in a similar way by increasing the contrast of adjacent pixels.

Once your image is open, zoom in to 100% or go to View > Actual Pixels. This is the most accurate way to view your image when making any fine adjustments. Other view sizes either drop or stretch pixels. When you view an image at 100% each image pixel takes up one screen pixel.

  1. Go to Layer > Duplicate Layer to duplicate your background layer.
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  2. With the new layer selected in the layers palette go to Layer > Smart Objects > Convert to Smart Objects. Your layer is now a smart object.
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  3. Go to Filter > Sharpen > Unsharp Mask.
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  4. The Unsharp Mask dialog box will appear.
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The Amount slider is where you specify the degree to which you want to sharpen the image. The Radius slider determines the thickness of the sharpened edge. A low number will produce crisp edges while a high number will produce thicker edges with more contrast throughout the image. The Threshold slider controls how Photoshop recognizes edges in an image. The number indicates the brightness values of two neighboring pixels that must occur if Photoshop is to sharpen those pixels. Low numbers sharpen a lot of pixels; high numbers exclude most pixels.

As a general rule, we want to sharpen the most pixels possible without dramatically increasing image contrast. So, as a starting point, set the Radius to 0.8 and the Threshold to 0. Then drag the Amount up until you see the image appear to sharpen. It is best to take the amount up too far and then bring it back down. BE AWARE OF OVER-SHARPENING. Too much sharpening will not only increase the contrast of your image, it will add digital noise and make your image look unnatural. Once you click OK your adjustment will be saved within the smart object layer. To fine tune this adjustment simply click on the unsharp mask adjustment within the layer and the dialog box will reappear with your most recent settings.

Once you are satisfied with your sharpening, it’s time to “flatten” your image for further adjustments in contrast, color, etc. This is done from the Layer menu, using either “Merge Visible” or “Flatten Image” below.


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