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Sharpening

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  #1  
Old 08-12-2004, 02:53 PM
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Sharpening

There are several ways in Photoshop to sharpen an image. The most commonly used are Unsharp-Mask (USM) and High-Pass filter.

USM Filter Basics: Sharpening increases the contrast at the edges of an image, ie, makes the dark pixels darker and the light pixels lighter. USM has three controls to adjust:
Amount: The higher the amount the stronger the sharpening. A small radius will need a higher amount setting. Katrin Eismann in her book Photoshop Restoration and Retouching suggests 120%-200% for offset printing and 40%-80% for direct digital output devices. For web or screen output, just eyeball it. As you increase the Amount parameter by moving the slider to the right, dark areas get darker and light areas get lighter. Do too much and you will end up sacrificing good pixels resulting in white halos and areas of solid black without any detail.

Radius: Radius determines how many pixels out from the color edge will have their contrast increased controls how much blurring is used in the edge-finding stage. Katrin Eismann suggests an accepted rule of thumb for offset printing is to divide the printer’s output resolution by 200 and use that as your starting point. As one web site puts it: "Radius is sort of like a brush size for the filter. A low setting limits sharpening to exact edges. A higher setting spreads the sharpening effect out according to the chosen radius."
This is the critical setting

Threshold: Adjusts how different the colors on the two sides of an edge have to be before the filter will recognize it as an edge and sharpen it. For example, a threshold Value of 5 means all tones within five value levels of 5 will be ignored.

Last edited by T Paul; 08-12-2004 at 03:47 PM.
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Old 08-12-2004, 02:54 PM
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USM Settings

There are many suggested settings for all three sliders and they will vary from photo to photo. Your best bet is to experiment and find what you like. Generally set the amount between 50% and 150%, the radius between 1-4, and the threshold between 3-20.

Suggested Settings: Possible starting points (these will differ from photo to photo and be dependent on the photo's size, resolution and so on)
General
Amount 150%, Radius 1, Threshold 3

Soft Subjects
Amount 150%, Radius 1, Threshold 10 (works well for images of a softer nature like people and flowers)

Maximum Sharpening
Amount 65%, Radius 4, Threshold 3 (works well on out of focus photos, or photos that have a lot of well defined edges like buildings)

All-purpose Sharpening
Amount 85%, Radius 1, Threshold 4

Web Sharpening
Amount 400%, Radius 0.3, Threshold 0 (good for blurry web graphics)

Lighter Web Sharpening
Amount 200%, Radius 0.3, Threshold 0

Last edited by T Paul; 08-12-2004 at 03:48 PM.
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Old 08-12-2004, 02:56 PM
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USM Tips

Always sharpen at 100%

Experiment with selective sharpening vs sharpening the entire image

Sharpen your image as the very last step and avoid resizing or retouching after you have sharpened

Experiment with applying USM to individual channels

Convert your image to Lab Mode and sharpen only the L channel

NOTE: Running USM more than once at a lower amount can sharpen more smoothly than running it once at a setting twice as high.
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Old 08-12-2004, 02:56 PM
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TECHNIQUES: There are a lot out there. Here are a few I found while searching the web
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Old 08-12-2004, 02:57 PM
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Unsharp Mask with Fade to Luminosity

The greatest problem with USM is that it can produce color halos along high contrast edges, especially if you over sharpen. A remedy for this is to use the Edit/Fade command set to Luminosity right after applying the USM to make sure the sharpening affects the luminosity only and not the color in the image.
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Old 08-12-2004, 02:58 PM
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Unsharp Mask and History Brush Tool

In this technique you take a History snapshot before and after using the USM filter. Then use the History Brush tool to paint back and forth between the sharpened and unsharpened snapshots.

NOTE: Alternative technique is to use a layer mask
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Old 08-12-2004, 03:00 PM
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Unsharp Mask "Smart Sharpening" With Edge Mask

This method allows you to sharpen edges and not noise, pores, or out of focus areas. What you do is use the Channels palette to create an "edge mask" which isolates the edges of the image so you can create a selection out them for using the USM. Note, this technique is rather long so you may want to create an action for it.

1. Duplicate the original.

2. Go to Channels palette and find the channel with the most contrast, duplicate it as an alpha channel and name it Edge Mask.

3. Select Filter/Stylize/Find Edges on the alpha channel. Invert the image (Comm/Ctrl - I) so you have white edges on a black background. The white edges will determine the eventual selection.

4. Select Filter/Noise/Median with value of 1 or 2 to accentuate the edges.

5. Select Filter/ Blur/ Gaussian Blur with value of 2 - 4 to "feather" the eventual selection. The blurring will get rid of noise as well as well as prevent unnatural looking transitions between sharpened and unsharpened areas.
Bruce Fraser: "High-resolution images typically require a higher-radius blur than low-resolution ones, and close subjects with soft detail, such as head shots, typically need a higher radius than high-frequency images with lots of fine detail."
6. Return to Composite view (Comm/Ctrl - ~) and Load the alpha channel as an "edge mask" selection (Comm/Ctrl - click the channel).

7. Now use the Unsharp Mask filter on this selection of just the edges of the image.

You can also download an action to automate this smart sharpening process and/or fine tune the action to your own needs.

Actions
Smart Sharpening by John Brownlow

Smart Sharpening by Ben Bardill

Last edited by T Paul; 08-12-2004 at 03:51 PM.
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Old 08-12-2004, 03:00 PM
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Sharpen Highlights and Shadows Separately

Duplicate the original image twice and use one to sharpen the highlights and the second to sharpen the shadows.
Then use the Blend If layer options to blend the two layers.
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Old 08-12-2004, 03:01 PM
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Unsharp Mask in LAB Mode

This method is supposed to avoid color shifts when using the USM. Convert the image to LAB mode and use the Unsharp Mask filter on the L (Luminosity) channel.

For portraits, flower shots, and things of a softer nature use amount 150, radius 1 and threshold of 10 for a starting point.

For objects that have well defined edges use amount 65, radius 3 and threshold 2 or amount 65 radius 4, and threshold 3 for a starting point.

Then convert back to RGB. There is some difference of opinion about whether you can shift modes like this and not introduce "quantization errors" from the switch.
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Old 08-12-2004, 03:03 PM
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Two Pass Sharpening

This is a summary of a tutorial by Bruce Fraser which is a variation of the High Pass Sharpening technique in that it involves creating an edge mask from a grayscale version of the image to isolate the effect of the USM. The idea that when you sharpen an image you may cause highlights to become too white and lose detail for printing. So he suggests a two stage process, first a mild sharpening with an edge mask, and then sharpening for the specific output, offset, inkjet, etc.

First pass - a mild sharpening to compensate for the effects of digitalization.
1. Duplicate image.

2. Convert the duplicate to grayscale using the method of your choice. (See summary of techniques on this site).

3. Run Filter/ Stylize/ Find Edges.

4. Run a Filter/ Blur/ Gaussian Blur to help get rid of noise and soften the transitions between sharpened and unsharpened images. Invert the image so the edges are white.

5. Use Levels adjustment to exaggerate the contrast and make sure edges are white and noise is pure black. You want to make sure there is still some gray for the eventual "feathering" of the selection.

6. Load the mask as a selection. Hide the marching ants of the selection with Command/Ctrl - H, and run the Unsharp Mask filter with something Radius .4, Amount 120%, and Threshold 0. You just want an mild sharpening to compensate for original digitization with this first pass.

Second pass - Sharpen the entire image for the particular output device.
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