Image Sharpening

Advanced Photoshop Tutorials by Jay Kinghorn

Image sharpening is one of the most often used yet least understood tools in Photoshop. By understanding image sharpening, how it works and how and when to use it, sharpening can be an effective and invaluable tool for your image correction arsenal.

What is sharpening?

Image sharpening in programs like Photoshop, Lightroom and Aperture are sophisticated uses of a very simple visual illusion. Have a look at the illustration (below left). Which of the rectangles, A or B, contains a darker shade of gray?

Sharpening 1

Sharpening 2

By removing the transition in the middle of the illustration (above right), it becomes clear that the color gray in both A and B is identical. The color difference in the middle of the illustration causes our brains to “see” a difference between the two shades of gray even though none exists.

This “feature” of our visual system has been understood, and exploited, for hundreds of years. Artists have long added a light-colored paint stroke to one side of an edge, and a dark stroke to the other to increase the apparent contrast of an edge in their paintings.

This is exactly how Photoshop, Aperture and Lightroom sharpen your images—by lightening one side of an edge and darkening the other. What makes the effect so successful is the control with which these applications apply sharpening to the edges in your digital photos.

How Sharpening Works

Most sharpening tools have at least two controls—Amount and Radius. The Amount setting controls the intensity of the sharpening, or the contrast of the lightening and darkening effect, along an edge. The Radius setting controls the width of the lightening and darkening effect along an edge. Regardless of the software application you’re using, the Amount and Radius work on essentially these same principles.

The simplicity of the controls belies an underlying complexity. The settings you choose for each slider, however, are dependent on the image capture device, image size, image content and the eventual use of the image.

Types of Sharpening

In an attempt to control the variables in image sharpening, many photographers adopted a three-step sharpening process where separate passes of sharpening are applied at different stages in the workflow. The first stage, often called Capture sharpening, adds a light amount of sharpening in the raw conversion process to compensate for the slight amount of image sharpness lost as the photo is digitized.

Note: The term capture sharpening is a bit of a misnomer as it is not a good idea to add sharpening in-camera using your camera’s menu. More often than not, sharpening added in-camera will cause problems later in your workflow. The first pass of sharpening should happen in your raw converter, not your camera.

The second (and optional) round of sharpening, called Creative sharpening, selectively sharpens key lines and focus areas within an image. For example, in a portrait, a photographer may add sharpening to the eyes, smile and jewelry to grab the viewer’s attention.

The final pass of sharpening, Output Sharpening, aims to optimize image sharpness based on the final image size, output medium (print or web; glossy or watercolor paper) and anticipated viewing distance. Of the three passes, this stage requires the greatest attention on the part of the photographer. It is very easy to over or undersharpen images and miss the mistake until the finished print rolls off the printer or appears in the magazine. To the uninitiated, this may feel like a bit of a high-wire act, but never fear, this article will help walk you through the process and provide plenty of guidance on what clues to look for to make sure you never oversharpen your images. It is far better to undersharpen images, than to oversharpen them.

A Sharpening Workflow

To put theory to practice, let’s look at the workflow for sharpening an image. I selected this image because it contains many of the challenges associated with image sharpening. It has fine detail that sharpens up well yet has smooth tones and high contrast edges that would show the effects of excessive sharpening.

You can download the photo to follow along with the article. Click on the iceclimber.jpg image link and right click on the image to save to your desktop. The photo is provided at a lower resolution (800×600 pixels) for downloading ease. I’ll use the full-resolution file for the initial capture sharpening, then the lower resolution file for capture and output sharpening. You’ll be able to follow along with the article through these two stages

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Capture Sharpening

The goal of capture sharpening is to improve upon the sharpness of the original capture in preparation for additional image corrections. Capture sharpening is most often performed in your raw processing software and is a light pass of sharpening. It may be tempting to try and make your photo “really sharp” at this stage. Don’t. Oversharpening in the capture sharpening stage will cause problems later in the workflow.

Both Lightroom and Photoshop share the Adobe Camera Raw processing module. Although I’m working in Lightroom 2 for this stage, the sharpening controls work identically in Photoshop CS4 even though the layout differs slightly between the two applications.

Capture Sharpening In Lightroom

Lightroom’s capture sharpening tools are located within the Develop module in the Detail panel. Here, Lightroom gives you four controls for adjusting the sharpness of your photos. Before adjusting any of these controls, it is wise to click on the image to zoom to 100% and preview the most important detail in the image. This step allows you to accurately see the effects of your sharpening and catch any oversharpening errors before progressing to the next stage.

Each of the four sharpening controls in the Detail panel—Amount, Radius, Detail and Masking—has a specific purpose. The Amount slider, as discussed earlier, controls the intensity of the lightening/darkening effect along the edges of the image. The Radius slider controls the width of the lightening/darkening effect along edges. The Detail slider, a bit tricky to describe, controls how deeply Lightroom “digs” into the image to pull out detail. The Masking slider helps remove sharpening from areas of smooth tone, like skin tones or blue skies, which can be ruined by too much detail. Each control has a hidden preview window accessed by pressing the Alt/Option key while adjusting the slider. This hidden preview helps display the image attribute most affected by the specific slider. For example, the Radius slider gives you a preview of the edges within the image while the Masking slider give you a preview of the masked areas within the photo. These previews are incredibly useful for gauging the effect of your sharpening and for learning how each sharpening control affects your images.

1. To begin, leave the Amount slider at its default setting and begin adjusting the Radius while holding the Alt/Option key. This brings up the hidden edge preview. You’ll need to set the radius based on the width of the finest detail within the image. In the ice climber photo, I’m going to choose a low Radius setting to ensure his hair, eyelashes and weave of his shirt are sharpened effectively. If I chose a high Radius setting, the width of the lightening/darkening effect could easy become wider than the detail itself, thus robbing detail from the image.

Tip: As a rule of thumb, it is better to hedge your bets toward a lower radius than a higher one. Most sharpening errors, visible in a finished print, are caused by setting the radius too high, thus creating a halo along high contrast edges. More on this in the Output section.

2. To be conservative at this stage, I’ll set the Radius at its lowest setting of 0.5. Next, set the Amount to control the overall intensity of the sharpening. Again, use the Alt/Option key to display the hidden preview, a grayscale representation of the image.

3. Increase the Amount slider until the important detail within the image just begins to appear sharp. For each image, there is usually one section you can use as a sentinel to give you a true reflection of the effects of your changes. For the ice climber, I’m paying attention to his eye and eyelash. As soon as they began to appear sharp, I stopped increasing the Amount at a setting of 53.

4. The Detail slider is good at teasing out subtle image detail and can be either a powerful ally or a dangerous weapon. For this image, I want to pull out detail in the climber’s shirt, eyes and hair, but I don’t want to exaggerate the pores in his skin. Increasing the Detail slider makes the pores in his skin too visible, so I’ll leave the Detail slider at its default setting of 25.

I shot this photo with a very wide aperture setting of f/2.5 to blur the background as much as possible. This helps visually separate the climber from a highly detailed, and otherwise distracting, background. Since this is a theme I established in the camera, I want to make sure I’m not increasing the sharpness of the background during my three rounds of sharpening. During the capture sharpening stage, I’ll use the Masking slider to remove my sharpening from the background.

5. Once again, hold the Alt/Option key while adjusting the Masking slider to see the hidden preview. The black areas are the masked areas where sharpening will not appear. Increase the Masking slider until the areas that should remain smooth and soft are masked effectively and the detailed images are unmasked (white).

6. At this point, I perform one final check of the sharpening by toggling the Detail switch on and off to create a before and after preview of the sharpening. I’m looking for any obvious lines or halos created by the sharpening, or for any areas that appear unnaturally sharp. Given the moderate sharpening applied during the capture sharpening stage, hopefully you won’t see any of these problems. Still, it is always important to check, because if you miss an error at this stage you will have to return to the raw file to correct it. This can result in repeating a lot of work.

Creative Sharpening in Photoshop

Our eyes are naturally drawn to high-contrast edges, both in our environment and within a photo. By making key edges within a photo sharper than others, we give them visual prominence, effectively telling the viewer, “look here, this is important.”

1. Creative sharpening is best applied to the full-resolution file on a duplicate layer, using a layer mask to restrict sharpening only to specific areas. In this example I’ve duplicated the background layer by pressing CMD/CTRL-J to create a copy of the background layer. This will become my creative sharpening layer. For organizational purposes, I’ll give the layer a name to help identify it after making future corrections. Double-click on the layer name or choose Layer > Layer Properties to rename the layer.

2. From here, you have two sharpening options available to you within Photoshop—Unsharp Mask and Smart Sharpen. Unsharp Mask (USM) is Photoshop’s time-tested sharpening tool. Smart Sharpen is the newer, upstart sharpening tool. Both will make your image appear sharper. However, the one you choose is dependent upon the image content, image resolution and image source. Here are a few guidelines.

Use Unsharp Mask when…

• Your digital photo originated as a scan of a piece of film.
• Your photo contains image noise caused by shooting at a high ISO setting or from rescuing an underexposed image.
• Your photo has lots of smooth tones, like skin or sky, that you don’t want sharpened.

Use Smart Sharpen when…

• Your photo contains lots of fine detail like hair or grasses
• You’re sharpening a web-resolution image
• Your photo was shot at a low ISO setting, is noise free and created by a digital camera (preferably digital SLR)

Smart Sharpening excels at pulling out detail in your photos. Unfortunately, it can be too effective in this endeavor, exaggerating image noise or adding texture to smooth tones. Therefore, it helps to use the best tool for the task at hand and following the above suggestions can help guide your decision-making process.

For this image, I’ll elect to use Smart Sharpen for creative sharpening and Unsharp Mask for final output sharpening. In your workflow, you can use either sharpening tool for both creative and output sharpening. Once you gain familiarity with the strengths and weaknesses of both tools, you’ll instinctively choose one over the other for the task at hand.

Typically, I use Smart Sharpen when output sharpening a web-resolution image like this one, but I want to be sure I don’t add sharpening to the soft background. Unsharp Mask contains a feature Smart Sharpen lacks that is beneficial in this type of situation. Plus, it allows me to demonstrate both sharpening tools in this tutorial.

Creative Sharpening With Smart Sharpen

3. With the Creative Sharpening layer targeted in the Layers panel, open Smart Sharpen (Filter>Sharpen>Smart Sharpen). Set the Remove option at the bottom of the dialog to Lens Blur, which is better suited for sharpening photos from digital cameras.

Smart Sharpen offers two controls, Amount and Radius. For Creative Sharpening, be sure to use a small Radius setting, controlling the sharpening intensity through the Amount slider. Generally speaking, I find I use a smaller Radius and a higher Amount with Smart Sharpen than with Unsharp Mask. Even though the Amount and Radius sliders are common between the two sharpening options, the math used by Photoshop differs between the two, creating slight differences in their sharpening effects.

4. Set the Radius visually, choosing a setting that best accents the finest detail in the photo. A starting point for a Web resolution image like this one is between 0.3 and 0.5 pixels.

5. For the sample image, I selected a Radius of 0.5, based on the catchlights in the climber’s eyes, the sharpness in his eyes and eyebrows and the detail in his shirt.

6. You can preview the effects of your sharpening by pressing and holding within the Smart Sharpen preview window. This temporarily disables the sharpening within the preview window. This is useful when looking for unintended impacts from the sharpening, like halos or uneven patterns.

7. Next, set the Amount to achieve the desired sharpening intensity. You should aim to subtly bring out the detail in key areas of the photo. Since the sharpening is contained on a separate layer and will be brushed into the image at a moderate opacity, it is okay to set your Amount a little higher than you would for capture sharpening. For this image, I set the Amount to 124%, which initially oversharpens the image, but will suit our purposes well because we’ll brush in the sharpening at a decreased opacity.

Tip: To become accustomed to identifying the effects of oversharpening in your images, take note of the image at the above settings. The halo along the climber’s left arm, the odd texture in the zipper of his jacket and the jagged lines along the strap on his helmet are all signs of oversharpening.

8. Press OK to apply the sharpening to the creative sharpening layer.

9. Since Creative Sharpening is intended to be applied only to key areas, your next step is to add a layer mask to the creative sharpening layer and fill the mask with black to hide the sharpening. Click on the Add Layer Mask button at the bottom of the Layers panel to add the mask and press Command/Control-I to invert the layer mask from white to black. This hides the sharpening. You’ll paint it back into key areas in the next step.

10. Select your brush tool (B) and choose a medium-sized soft brush with a low Hardness setting. For this image, I recommend a 30-pixel brush, a hardness setting of 0 and a brush Opacity around 50%.

11. Set white as your foreground color by pressing D on your keyboard, then brush the sharpening into key areas of the photo. In this image, I brushed sharpening into the climber’s eyes, his hands, the lit suspender and very lightly on the logo on his sleeve.

Output Sharpening

Sharpening your photo for output is the final step before sending the photo to the printer, uploading it to a client or posting it on the Web. Altering the image size often negates the effect of sharpening. As a result, you should always perform your output sharpening on your photo at the final image size and resolution.

Output Sharpening In Lightroom

One of the advantages of working in Lightroom is the built-in output sharpening presets found in the Export and Print modules. These presets are surprisingly good and take much of the guesswork out of output sharpening. These presets are fairly self-explanatory. Simply choose the appropriate media type and the sharpening intensity; low, medium or high. Lightroom takes care of the rest.

Output Sharpening In Photoshop

Sharpening your image in Photoshop is still a manual process. Depending upon the output type, your image size and image content, you can use either Smart Sharpen or Unsharp Mask for your output sharpening. I typically use Smart Sharpening for Web or screen-based output and favor Unsharp Mask for printing, though the image content is what ultimately affects the final decision. For this example, I’ll use Unsharp Mask to prepare an image for printing on a semi-gloss inkjet paper.

Note: Typically, when sharpening an image for print, I recommend setting your image’s zoom setting at 50 percent. This gives a better preview of how the sharpening is being applied to the image as a whole. Keep the preview window within Smart Sharpen or Unsharp Mask at 100 percent to see how the sharpening is affecting the key details in your image. This gives you both the macro and the big-picture view of your image.

For the purposes of this tutorial, keep your zoom setting at 100 percent since this is a Web resolution image.

1. I like to keep my final output sharpening on a separate layer above any retouching or adjustment layers. To do this, single click on the topmost layer in the layer panel, then pressCMD-Option-Shift-E on the Mac, or CTRL-Alt-Shift-E on Windows. This merges all visible layers into a new layer. Rename this layer “Output Sharpen” and change the layer blending mode from Normal to Luminosity. This ensures your sharpening is applied to the luminance, or tone, information and not to the color information. This helps retain the integrity of edges and ensures sharpening does not increase the appearance of lens problems, like chromatic aberration.

2. Access the Unsharp Mask (USM) command via the Filter > Sharpen > Unsharp Mask menu. Like Smart Sharpen, USM contains an Amount and Radius slider used to control the width of the sharpening halo and the intensity of the sharpening. USM also contains the Threshold slider which controls how the sharpening is applied.

3. Set your Radius based on the finest detail in the image. When sharpening for print, you can use a slightly higher setting than when Creative sharpening. For this image, a Radius of 1.0 works well. Any higher and the halo along the side of the climber’s face and left arm would be easily visible in the finished print.

4. Given the inherent sharpness of the image, from the initial capture and the two prior passes of sharpening, I can choose a low Amount setting of 57 percent. For a Web image, I’d choose a slightly lower setting, but since this is an image destined for print, I’m expecting the sharpness to dull slightly when the ink is absorbed by the paper. On a watercolor paper, I’d push the Amount a little higher since the paper is highly absorbent.

5. At this point, you can consider your sharpening complete, press OK and send your photo to the printer. However, before I would send this photo off, since I have a nicely out-of-focus background, I want to take one insurance step to ensure the sharpening I’ve added is applied to the sharp areas of the photo, and not to the background. This is where the Threshold command becomes extremely valuable.

6. The Threshold command controls how similar two adjacent pixels need to be in order to have sharpening applied. At a Threshold setting of 10, any pixels with a pixel difference of greater than 10 would be sharpened, while two pixels in a smooth area with a pixel difference of 8 would not. The best way to see the effect in action is to increase the Amount and Radius significantly and begin increasing the Threshold slider. Smooth areas of the image will lose sharpening and return to their unsharpened state.

When this image (above) is sharpened excessively, you can see how a Threshold setting of 15 removes the sharpening from the smooth areas within the photo.

Setting the Threshold at 7 helps ensure the smoothest areas within the photo will not be sharpened even though the effect is very difficult to see.

7. Press OK to apply the sharpening. Your image is now ready for printing.

Conclusion

These techniques should help you become more comfortable with the sharpening process. Of particular importance is recognizing the effects of oversharpening before you progress too far in the workflow. The three-pass sharpening method is a solid approach to incorporating sharpening at various stages within your workflow, though some photographers opt for a single pass of sharpening at output. Ultimately, choose the one you feel gives you the best results and strikes an effective balance between your time investment and quality output.

The more often you sharpen your photos, the more you will develop an intuitive sense for how much sharpening to apply. These techniques give you a solid foundation for exploration. I hope they find a home in your image processing workflow.