Basic Sharpening in Photoshop

Unsharp Mask

There are several ways to sharpen in Photoshop, the most useful of which is usually the Unsharp Mask filter (Filters, Sharpen, Unsharp Mask.)

Threshold

The unsharp mask tool has three parameters, Amount, Radius and Threshold. The threshold setting is used to set where in the image the sharpening will occur. At zero the whole image will be sharpened. Otherwise the value sets a minimum level between adjacent pixels; if the difference in values is less than this, sharpening will not occur. For most purposes a value 0-10 is used, but higher values will help to avoid the creation or emphasis of unevenness in smooth areas of tone.

Radius

Radius is used to set the amount and extent of the sharpening. We can set decimal fractions values, such as 0.3, in this box, although when sharpening an image for print output, rather larger values will be needed. Overlarge values will give visible halo effects around edges. A small degree of halo formation is essential to get the sharpest possible effects in print (either from inkjet printers or from repro) but should be avoided if possible for work on screen.

Amount

The Amount setting largely seems to act as a multiplier for the radius. If you are using the Unsharp Mask filter for 'creative sharpening' it will most normally be in the range 60-150%, but when applying 'output sharpening' for print you may need rather higher values.

Unsharp Mask: Luminance

Some people prefer to use unsharp masking not on the RGB image, but to convert this to a LAB image and sharpen only the lightness channel. The image is then converted back to RGB. This can avoid the problem of colour artifacts which are sometimes generated when sharpening.

Unsharp Mask with Edge Mask

Even with careful tweaking of the Threshold value, unsharp masking tends to increase noise in areas of relatively smooth tone. Generally you want to sharpen the edges while leaving these areas unchanged. This can be achieved by selecting the edge areas with a suitable selection mask before applying the unsharp mask filter.

1.       Duplicate the image, using 'Image', 'Duplicate', accepting the default file name.

2.       Change the duplicate to Gray Scale, using Image, Mode, Grayscale

3.       If the image is 16 bit, change to 8 bit, using Image, Mode, 8 bit

4.       Find the edges using Filter, Stylize, Find Edges.

5.       Open the levels dialogue (Ctrl+L) and shift the white-point marker at the right under the histogram to the left until only edges are shown. You can also move the black point marker to the right to make the black edges more definite. The OK to apply

6.       Use Filter, Blur, Gaussian Blur, adjusting the radius to keep the edges clearly visible but with soft edges - typically around 2 pixels, OK to apply.

7.       Invert the image using Ctrl + I

8.       Open the Levels dialogue (Ctrl+L) and again shift the white point marker to get clear whites, OK

9.       Return to the original image, and 'Select, Load Selection' accepting the default name as a new selection.

10.    Press Ctlr+H to hide the 'marching ants' and then use Filters, Sharpen, Unsharp Mask. For 'Creative Masking' choose relatively small values of radius, perhaps around 0.5-2 pixels, keep the threshold at zero and adjust the amount to get sharpness without significant halos. For Output sharpening for printers, try values of 1.5-4 pixels for the radius, viewing the image at 25% and making the effect slightly too noticeable on screen. OK to apply.

It would be better practice to carry out the sharpening on a layer, but this would double the final file size.

Unsharp Mask Filter

About the Photoshop sharpening filters

Use the Unsharp Mask (USM) filter to sharpen your images. The controls provided by this filter make it the best choice for sharpening. Although Photoshop also has the Sharpen, Sharpen Edges, and Sharpen More filter options, these filters are automatic and do not provide controls and options. You might use these filters for producing creative effects in your image.

Photoshop lets you sharpen your entire image or sharpen part of your image by using a selection or mask. Since the Unsharp Mask filter can only be applied to one layer at a time, you might need to merge layers or flatten your file to sharpen all image layers in a multilayered file.

Note: The name, Unsharp Mask, makes the filter seem like an unintuitive choice for sharpening an image. The term comes from a darkroom technique used in traditional film-based photography. The effect of a sharpened edge can be produced by sandwiching a photographic negative and a slightly blurred and underexposed copy negative in the enlarger. Then the photographic paper is exposed through the sandwiched negatives.

Using the Unsharp Mask filter

The Unsharp Mask does not detect edges in an image. Instead, it locates pixels that differ in value from surrounding pixels by the threshold you specify and increases the pixels' contrast by the amount you specify. So, for neighboring pixels specified by the threshold, the lighter pixels get even lighter and the darker pixels get even darker based on the specified amount.

In addition, you specify the radius of the region to which each pixel is compared. The greater the radius, the larger the edge effects.

Original image and Unsharp Mask applied

The degree of sharpening applied to an image is often a matter of personal choice. However, over-sharpening an image produces an unattractive halo effect around the edges.

Over-sharpening an image produces a halo effect around the edges.

The effects of the Unsharp Mask filter are more pronounced on-screen than in high-resolution output. If your final destination is print, experiment to determine what settings work best for your image.

To use Unsharp Mask to sharpen an image:

1. (Optional) If your image is multilayered, select the layer with the image you want sharpened. The Unsharp Mask can only be applied to one layer at a time, even if layers are linked or grouped. You can merge the layers before applying the Unsharp Mask filter.

2. Choose Filter > Sharpen > Unsharp Mask. Make sure the Preview option is selected.

Click the image in the preview window and hold down the mouse to see how the image looks without the sharpening. Drag in the preview window to see different parts of the image, and click + or - to zoom in or out.

Although there is a preview window in the Unsharp Mask dialog box, it's best to move the dialog box so you can preview the effects of the filter in the document window.

3. Drag the Radius slider or enter a value to determine the number of pixels surrounding the edge pixels that affect the sharpening. The greater the radius value, the wider the edge effects. And the wider the edge effects, the more obvious the sharpening.

Adjusting the Radius value depends on the subject matter of the image, the size that the image will be reproduced at, and output method. For high-resolution images, a Radius between 1 and 2 is usually recommended. A lower value sharpens only the edge pixels, whereas a higher value sharpens a wider band of pixels. This effect is much less noticeable in print than on-screen, because a 2-pixel radius represents a smaller area in a high-resolution printed image.

4. Do one of the following:

• Drag the Amount slider or enter a value to determine how much to increase the contrast of pixels. For high-resolution printed images, an amount between 150% and 200% is usually recommended.
• Drag the Threshold slider or enter a value to determine how different the sharpened pixels must be from the surrounding area before they are considered edge pixels and sharpened by the filter. For instance, a threshold of 4 will affect all pixels that have tonal values that are different (on a scale of 0 to 255) by 4 or greater. So, if adjacent pixels have tonal values of 128 and 129, they will not be affected. To avoid introducing noise or posterization (in images with fleshtones, for example), use an edge mask or try experimenting with Threshold values between 2 and 20. The default Threshold value (0) sharpens all pixels in the image.

If applying Unsharp Mask makes already bright colors appear overly saturated, convert the image to Lab mode and apply the filter to the Lightness channel only. This sharpens the image without affecting the color components.

Selective sharpening

You can sharpen part of your image instead of the entire image by using a mask or a selection. This is useful when you want to prevent certain parts of your image from being sharpened. For example, you can use an edge mask with the Unsharp Mask filter on a portrait to keep the facial skin soft looking but sharpen features like the eyes, mouth, nose, and outline of the head.

Using an edge mask to apply the Unsharp Mask only to specific features in a portrait

To sharpen a selection:

1. With the image layer selected in the Layers palette, draw a selection.

2. Choose Filter > Sharpen > Unsharp Mask. Adjust the options and click OK.

The image area within the selection will be sharpened, leaving the rest of the image untouched.

To selectively sharpen an image using an edge mask:

1. Create an edge mask.

There are many ways to create an edge mask. Experiment with the following method:

• Open the Channels palette and select the channel that displays the grayscale image with the greatest contrast in the document window. Often, the green or the red channel will be the one you select.

Showing a channel with the greatest contrast

• Duplicate the selected channel.
• With the duplicate channel selected, choose Filter > Stylize > Find Edges.
• Choose Image > Adjustments > Invert to invert the image.

Find Edges filter applied and image inverted

• With the inverted image still selected, choose Filter > Other > Maximum. Set the radius to a low number and click OK to thicken the edges and randomize the pixels.
• Choose Filter > Noise > Median. Set the radius to a low number and click OK. This averages the neighboring pixels.
• Choose Filter > Blur > Gaussian Blur to feather the edges.

1. Note: The Maximum, the Median, and the Gaussian Blur filters soften the edge mask so the sharpening effects will blend better in the final image. Although this procedure recommends using all three filters, you can experiment using only one or two.

• Choose Image > Adjustment > Levels and set the black point high to get rid of random pixels. If necessary, you can also paint with black to retouch the final edge mask.

Set the black point high in Levels to eliminate random pixels in the edge mask

2. In the Channels palette, Ctrl-click (Windows) or Command-click (Mac OS) the duplicate channel to make the edge mask a selection.

3. In the Layers palette, select the image layer. Make sure the selection is visible on the image.

4. Choose Select > Inverse.

5. With the selection active on the image layer, choose Filter > Sharpen > Unsharp Mask. Set the desired options and click OK.

Note: You can create an action to conveniently apply all the steps in the procedure.

Sharpen a photo

How To Tip:

Sharpen a photo using the Unsharp Mask filter.

1. In the Layers palette, select the layer containing the image you want to change.

If you want to sharpen only a specific element or area in the image, create a selection marquee around the area you want to sharpen using the selection tools. The selection marquee limits the changes to that area. To soften the edge of the selection so the changes blend in more naturally with the unselected areas, use the Feather command (Select > Feather).

2. Choose Filter > Sharpen > Unsharp Mask.

3. In the Unsharp Mask dialog box, make sure Preview is selected. Then position the Unsharp Mask dialog box so that you can see most of the image.

4. Drag the Amount slider to the right to increase the contrast of the edge. For high-resolution printed images, an amount between 150% and 200% is usually satisfactory. Grainy images usually require a lower setting.

5. Drag the Radius slider to the right to increase the width of the edge created by sharpening. If you set this too high, the edges will acquire obvious halos. A setting between 1 and 2 is often adequate.

6. Drag the Threshold slider to the right to increase the difference needed between shades before the filter sharpens the edge between them. A threshold of zero sharpens all pixels in the image. A high threshold number sharpens the edges only between significantly different shades. Values between 2 and 20 are usually acceptable.

7. Experiment with these three controls until you are satisfied with the results, and then click OK.

You can let Photoshop sharpen the image for you using the three other Sharpen filters. Choose Filter > Sharpen, and then choose one of these filters from the submenu: Sharpen, Sharpen Edges, or Sharpen More.