Photoshop is adept at detecting “edges.” By which I mean, areas in your photographs where luminance levels shift rapidly. When Photoshop spots an edge, it can enhance its contrast, giving the impression of greater sharpness. What sharpening can’t do, of course, is go back in time and readjust the focus in your camera. In other words, it can’t fix blurry images. But, what sharpening can do is take a well-focused image and make it look even crisper. In this in-depth article, we’ll look at the fundamentals and core techniques of sharpening.
It might surprise you to know that there are five Sharpen commands in Photoshop’s Filter menu alone—not to mention related filters such as High Pass and Emboss—making sharpening the most predominant filter effect in the application. Photoshop’s wide array of sharpening options gives you an enormous range of control and flexibility.
In this article, we’ll look at the fundamentals and core techniques for sharpening images—specifically, how to apply the right amount of sharpening while minimizing noise and other artifacts.
Photoshop sees edges whenever the luminance values change rapidly between pixels in close proximity. In other words, if the pixels in one area have a certain brightness and just a few pixels away are a group of pixels that are either much darker or much lighter, Photoshop will treat that as an edge. Sharpening enhances such transitions by making the light side of the edge slightly lighter and the dark side slightly darker.
Our eyes interpret the amount of contrast on an edge as its degree of sharpness. What happens when you sharpen out-of-focus areas? Instead of bringing elements into focus, it can harden edges that are supposed to be soft and emphasize noise, because noise consists of random variations in luminance.
Under the Filter > Sharpen > menu, you’ll find the Sharpen, Sharpen Edges, and Sharpen More commands.
These are easy to use. You simply select a layer to apply them to, select the command, and let it do its thing. The results are not terribly satisfying, but they are useful for web work. You’ll want to resample your image to the appropriate size and view at 100-200% before applying the commands to see the results more clearly.
- None of these three methods is robust enough to survive printing.
- Sharpen produces an extremely subtle effect. Deke never uses it.
- Sharpen Edges produced an even more subtle effect than Sharpen. It actually applies the same amount of sharpening, but it ignores some dust, noise, etc.
- Sharpen More results in substantially more sharpening on screen.
Controlled sharpening with Unsharp Mask and Smart Sharpen
The one-shot sharpeners owe both their ease of use and their limited usefulness to the fact that they don’t have any controls. For that reason, Unsharp Mask and Smart Sharpen are the go-to sharpening commands for sharpening. They are much more useful because you can control them.
If you learn nothing else about sharpening in Photoshop, you’ll want to learn how to use Unsharp Mask, or USM for short. It is the primary sharpening tool inside Photoshop, and it’s the first sharpening filter you need to learn.
USM gets its name from a traditional darkroom technique that has been used with still images and movies since the 1930s. The digital version uses Gaussian Blur to create a blurred (unsharp) copy of the image which is then applied to the original as a kind of mask. That fuzzy mask produces haloes around the edges in the image, which gives the impression of increased sharpness.
Typically, you’ll want to apply sharpening while observing your image at output size. If you’re sharpening for the web, that means resampling the image to the proper size and then viewing at 1% (Ctrl/Command +1). If you’re sharpening for print, you’ll want to make sure Photoshop knows the correct resolution for your monitor (it’s not 72 dpi), and then select View > Print size from the menu bar. We’ll look at this in more detail later in this article.
You apply Unsharp Mask to a pixel layer. If your background layer is the only layer, you can duplicate it. If you have lots of layers, you can stamp a layer: add a blank layer at the top and then hold the Alt/Option key down while selecting Merge Visible from the fly-out menu at the upper right corner of the Layers panel.
To apply the USM filter, select Filter > Sharpen > Unsharp Mask from the menu bar. When the dialog box appears, you can position it so that you can view as much of the image area as possible.
- The dialog contains a small preview area. You can click in the main image to show the corresponding area of the image in the preview area inside the dialog.
- You can also drag inside the preview area to reposition its contents.
- Place the cursor inside the preview area in the dialog and hold the mouse button down to show the image without sharpening. Release the mouse to show sharpening again.
- Toggle the Preview checkbox to show or hide the effect of the sharpening settings in the main window.
Set the amount: This typically ranges between 50%–200%.
Set the Radius: This value effects the thickness of the haloes that appear around edges. The higher the radius, the more blur you’re applying. Generally, you’ll want to reduce the radius to create a sharp, tactile effect.
- Using a high Amount with a low Radius gives a sharpening effect. A radius of around 3 generally works best for printing, while you’ll use a radius as low as .5 for screen.
- A low Amount with a high Radius can create a volume enhancement or clarity effect. Shadows and highlights can be boosted dramatically.
Sharpening can make noise stand out, particularly as you increase the Amount setting. You can use the Threshold setting to tell the filter to ignore details that have less pronounced edges. The value indicates the minimum luminance difference between adjacent pixels needed for the effect to kick in.
- In most cases, you’ll leave the Threshold set to zero.
- Click in the field and use the up arrow key to nudge the value until the texture disappears.
- Use a value of 1-3 levels to remove noise.
- High Threshold values create an unpleasant pock-marked effect.
- Never use Threshold for sharpening on someone’s face. It really produces bad results. (In fact, sharpening skin generally brings out unwanted texture.)
The Color Fringe Problem
All of the sharpening filters in Photoshop, including Unsharp Mask have a habit of producing aberrantly colored haloes. In each case, this problem happens for the same reason: Photoshop generates haloes on a channel by channel basis and then combines the results. Whenever you have an edge area that has a pronounced color, you’re likely to see the tones reverse in one of the channels, and that creates a color shift.
I the example above, the tones in the Blue channel are roughly the inverse of those in the Red and Green channels for the part of the image we can see. A light halo appears on the inside of the insect’s head in the Blue channel, while the halo is dark in the other two channels. This results in a blue fringe surrounding the head and blue antennae. You can also see red and yellow flares around the darker area at the upper right.
The solution: Use the Fade command to blend the sharpening effect with the original. In every sharpening operation you do, you should always fade to Luminosity.
- Immediately after you apply USM, select Edit > Fade Unsharp Mask from the menu bar or use the keyboard shortcut Ctrl/Command + shift + F
- The Fade command dialog box will appear
- Change the blend mode to Luminosity
- Optional: Adjust the Opacity. You can toggle the Preview checkbox to evaluate the result.
- Click OK to apply the fade.
Note: Fade is only available immediately after you apply the filter.
Gauging ideal sharpening for web and mobile devices
For the web, you’ll typically use a high Amount and a very low Radius.
- View your image at 100%. You can use Ctrl/Command + 1 or select View > Actual Pixels from the menu bar.
- Start with an Amount of 500%
- Set the Radius to as low as 0.3 px - 0.7
- Increase Threshold as needed. (In this example, we went to 5 levels.)
- Readjust the amount
Tell Photoshop your screen resolution
In the next section, we’ll look at sharpening for print. To do that effectively, you’ll want to view the image at print size, and to do that, Photoshop needs to know your screen resolution. Since it can’t ask the monitor directly, you’ll need to do some measuring and a little math to enter the right value.
If you select View > Print Size from the menu bar, your image will probably appear conspicuously smaller than it should be. That’s because Photoshop assumes your monitor has a 72 ppi resolution. This is a relic — the days of 72 ppi screens are long gone, and most displays now range between 96 and 120 ppi.
Here’s how to determine your monitor’s actual resolution:
- Measure the width and height of the image area, excluding any black areas surrounding the screen. You just want the part that actually displays something. (You really only need one dimension, but it’s useful to know both dimensions.) The active screen area of a 27” iMac measures 23.5” wide.
- Now get the pixel dimensions of your screen and write them down. For the iMac, the screen is 2560 x 1440 pixels.
- On Windows: Right-click on the Desktop and choose the Properties command.
- On Mac: If the Displays icon appears in your menu bar, just pull it down and look at the dimensions. - or -
- Choose System Preferences from the Apple menu and click Displays in the main dialog. When the Displays panel appears, click Display in the navigation bar at the top of the dialog to show the resolutions list. The selected resolution will appear in the list.
- To calculate PPI, simply divide the number of screen pixels by the physical inches. You can do the calculation with either the width or the height. For the iMac example, that works out to 256px ÷ 23.5” = 109 ppi.
Now, you can enter that value into the Photoshop preference settings:
- Select Preferences > Units & Rulers from the menu bar.
- Locate the New Document Preset Resolutions section.
- Enter your PPI value into the Screen Resolution box.
- Click OK.
Once you’ve entered an accurate value into the preference, your document will appear at the right magnification when you select View > Print Size from the menu bar.
Gauging ideal sharpening for print
View > Print size = Ctrl + Alt + Command + Option + 0
Start with amount at 500%
You need at least 1 px of radius for every 3 px of resolution. So, 3 px is great for 3 ppi output.
Take radius up to ~ 3 px. Looks like way too much sharpening going on… You need a higher radius value for printing.
Reduce the amount until it looks good, then add back 50%. (if 100% looks good, go with 150%)
Edit > Fade mode = luminosity.
Reapplying the last-used filter
The last-used filter appears at the top of the Filter menu. So, if you just ran Unsharp Mask, it appears in the menu, ready to be reused, with a caveat. The menu item is designed to bypass the filter dialog and re-run the last filter with the same settings. You can force the dialog to open with the previous settings by holding down the Alt/Option key as you select the menu item.
The menu item also has a keyboard shortcut, which can be used with or without the Alt/Option key:
- Reapply the last filter: Ctrl/Command + F
- Open the last filter dialog: Alt/Option + Ctrl/Command + F
Using Smart Sharpen
Smart Sharpen is mostly an upgrade to Unsharp Mask. It lacks a Threshold control, but gives you controls and options that USM lacks. The Smart Sharpen dialog is significantly larger than the USM dialog. The big preview is not particularly useful, since the image, at proper resolution, sits just behind.
To open the dialog, select Filter > Sharpen > Smart Sharpen from the menu bar.
The Remove menu beneath the Amount and Radius sliders has three methods: Gaussian Blur, Lens Blur, and Motion Blur. Each method creates a different sharpening effect.
Remove: Gaussian Blur
With Remove set to Gaussian Blur, Smart Sharpen is doing the same thing as USM. However the dialog does not have a Threshold option, so you end up sharpening more noise. The implication is that Smart Sharpen is best used with low-noise images.
- Smart Sharpen / Gaussian is great for dealing with the effects of downsampling (e.g. using the Image Size command with resampling to shrink the image.)
- Follow up downsampling with Smart Sharpen / Gaussian at a low Amount setting (100% or less) and a small radius (0.3-0.5px).
- Smart Sharpen / Gaussian is also good for scanned images.
Remove: Lens Blur
Sharpening with Remove set to Lens Blur gives you much better edges. It’s a subtle, but meaningful difference. The first difference you’ll notice between Gaussian and Lens Blur is that the Lens Blur halo is smaller than with Gaussian Blur at the same Radius setting. To keep the same halo size, you need to increase the radius setting on the Lens blur by another half. In other words, with Lens Blur, you’ll need to set the Radius to 4.5 pixels to produce a halo equivalent to the 3 pixel setting with Gaussian Blur.
Good Lens Blur Radius settings:
- Print: ~4.5 px
- Web: ~1 px.
Lens Blur produces sharper, better details than USM/Gaussian Blur. The higher the resolution of the file, the greater the difference.
I can only give you qualitative statements in this tech note, but if you watch the video, Deke will actually give visual demonstrations of the differences between sharpening methods, particularly in terms of how they deal with edges.
Remove: Motion Blur
Sharpening with Remove set to Motion blur is designed to correct for the effects of minor camera shake. We’ll go into detail about this setting in the next article.
When you hover your cursor over the More Accurate checkbox, the tool tip says “Toggle to produce a more accurate sharpening effect,” which certainly sounds appealing. You can chalk that one up as false advertising. The feature does not produce a more accurate effect; what it produces is a much busier effect—the filter essentially becomes O.C.D. The check box is off by default for good reason, and should be off for most images.
With the box checked, Photoshop doubles up the tracing around edges and also traces around individual bits of noise (or skin texture). Because it emphasizes texture, it can be useful with low-noise still-life subjects.
- Exaggerates noise.
- Emphasizes pores and defects in people’s skin. (Do not use this option if you’re sharpening a portrait.)
- What “More Accurate” is good for: If you’re trying to bring out fabric, wood grain, or even the scales of a slow-moving animal, it can be useful.
There is a quirk (bug?) in the way the settings menu works inside the Smart Sharpen dialog. In dialogs such as the Color Preferences, and even in panels such as the Curves display in the Adjustment panels, you can select a preset, and when you change the settings, the menu switches to read “Custom.” In Smart Sharpen, the menu starts out reading “Default” and never changes unless you select something. To make matters more interesting, the dialog stores the current settings in whatever menu item is selected when you click the OK button, and that can be a problem.
If you’ve already used Smart Sharpen, chances your “Default” settings are no longer the default settings that shipped with Photoshop. We’ll create a useful default first, then look at how you can manage settings going forward.
- Select a layer to apply Smart Sharpen to
- Select Filter > Sharpen > Smart Sharpen to open the SS dialog.
- Enter the Smart Sharpen defaults: Amount = 100%, Radius = 1px, Remove = Gaussian Blur, More Accurate Off.
- Click OK to apply the filter and immediately undo with Ctrl/Command + Z
Now, let’s create a meaningful default for printing.
- Select Filter > Sharpen > Smart Sharpen to reopen the SS dialog.
- Enter your settings: Amount = 250%, Radius = 4px, Remove = Lens Blur, More Accurate Off (Lens Blur Remove is the real option for producing more accurate results.)
Before applying the filter, let’s create a preset:
- Click the floppy disc icon to open the save dialog.
- Enter a name for your settings.
- Click OK to save the settings.
- Note that the menu still reads Default. Do not click OK, or your settings will be saved in the Defaualt preset!
- Select your newly-saved preset (e.g. Screen Sharpen Stronger) from the Settings menu.
- Click OK to apply the filter and save the settings into the current Settings menu item.
The moral of the story:
You can select an item from the Settings menu to establish starting parameters for your sharpening and then revise the settings ad needed. But, be careful to save the new settings with a different name and select that item from the menu before you click OK to apply the sharpening. Otherwise, your default settings will be overwritten. If you don’t want to fill your menu with items for every single variation you create, re-save a setting called “Last Used” and be sure to select it before you click OK to apply the sharpening. Photoshop will ask you if you want to replace the settings with the same name. Click OK to confirm.
Using the Advanced option
Enhancing edges by brightening the light side and darkening the dark side means you’re running the risk of clipping strips of highlights and shadows throughout your image, especially when either the Amount or Radius setting has a high value. When you click the Advanced radio button in the Smart Sharpen dialog, it activates a set of tabs with extra features that give you a bit more control over shadows and highlights.
The Sharpen tab contains the same basic controls and behaves the same way it did in the previous examples.
In the Shadows and Highlights tabs, Deke recommends you use the exact same settings for both:
- Fade Amount controls clipping. Use 100%.
- Tonal Width defines what tones are considered shadow or highlight, tapering off to either side of 50% gray. Leave it at 50%.
- The Radius setting constrains where the fade is applied, but it is an extremely subtle change, even at a radius of 100 px. Use the default—1 px.
As mentioned earlier, once you have defined your settings, DON’T CLICK OK to apply the sharpening until you save a preset in the Settings menu and select that item. Otherwise, you’ll replace the values in the currently selected settings menu.
Once the sharpening is applied, you’ll want to fade the effect:
- Open the Fade command (Edit > Fade Smart Sharpen or Shift + Ctrl/Command + F)
- Set the Mode to Luminosity
- Optional: Reduce the opacity to control the overall sharpening effect. Toggle the Preview check box to see the effect of the fade.
- Click OK to commit the fade.
So, there you have it — you now have the keys to the two most useful and versatile sharpening commands in Photoshop. You can use the Unsharp Mask command to sharpen edges and ignore noise with its Threshold control. Smart Sharpen is an alternative to Unsharp Mask that gives better edge definition with its Lens Blur Remove setting, though it lacks a Threshold control. Its poorly-named “More Accurate” feature can dramatically enhance textures, and its Advanced settings allow you to control clipping in highlights and shadows.
You can apply either sharpening filter at different strengths, depending upon whether you’re sharpening for screen or for print, and you can create reusable presets. Once you apply the sharpening, you’re not really done until you fade the sharpening to Luminosity mode to eliminate aberrant colors that any sharpening method is likely to create.
More in the next tip:
Unsharp Mask and Smart Sharpen can handle most of your sharpening needs, but there are several advanced techniques to look at.
- One is dealing with the subtle blurring of camera shake. We’ll look more closely at Motion Blur Remove in Smart Sharpen along with an alternative technique that can produce less noise.
- We’ll look at a different way to sharpen images and control clipping that does not involve the Sharpen filters.
- And, we’ll look at a Photoshop tool that has stealthily been given new life in Photoshop CS5 that allows you to selectively paint sharpening into areas of your image without filters or masks.