Unsharp Mask: How Do You Actually Use That Thing?
Until recently, if someone said the word “sharpening” to us, we’d whimper and hide under the table.
We mean, what the #$% is a threshold anyway?
Well, we finally got fed up with it, so we did some research. And you know what? Sharpening’s actually not that bad, and it makes a HUGE difference on digital images.*
Here’s our no-nonsense, jargon-free guide to sharpening your photos using Unsharp Mask. It’ll change your life. We promise.
*If you’re printing directly from film, feel free to be smug at this time. You don’t need to sharpen a darn thing.
p.s. Hey San Francisco! Wanna help us out with the Photojojo Book? We need people to photograph and places to photograph them in. Check out our wishlist! We’ll make ya book-famous, baby!
Why Do Digital Pictures Need Sharpening?
You never had to sharpen your photos when you were using film, so why do digital photos need it? Because film and digital cameras record images differently, young padawan. Read on…
Digital cameras have a fixed grid of pixels, each of which can only capture one color or shade at a time. Say you take a picture that has a sharp edge between black and white. The razor-thin boundary of that edge would look half black and half white to the human eye. But the single pixel that records that hairline edge can only record one color, so it renders it as gray.
What we think of as sharpness is actually the contrast we see between different colors. A quick transition from black to white looks sharp. A gradual transition from black to gray to white looks blurry. So when we look at the picture you just took of that sharp black & white edge, the gray pixels along the edge will make the photo look blurry.
Sharpening your picture increases the contrast along the edges where different colors meet. This tricks the eye into believing that the photo looks sharper, better, stronger.
What If I Don’t Want to Sharpen?
The good news is, most digital cameras include a sharpening feature. They sharpen your photos as part of the recording process, so you never see that blurry image at all. If you don’t want to worry about sharpening your pictures, make sure that feature is turned on in your camera, and you’re all set.
The bad news is, your camera isn’t as smart as you are and it may sharpen your pictures too much or too little. Plus, if your camera has already sharpened your images, you shouldn’t sharpen them again yourself. Twice-sharpened images just look crummy.
If you want the most control over your images, look for the “sharpen” or “sharpness” feature in your camera’s menu and turn it down as low as it will go. Turn it off if you can. Then you can control the sharpness yourself in Photoshop.
Some Basic Rules
Although there are no one-size-fits-all rules, we can give you a couple of rules of thumb.
- You can’t add detail that wasn’t already there. If the image was out-of-focus to begin with, sharpening won’t help. Sorry Charlie.
- Don’t sharpen until the last step of the editing process. Crop, make all your color adjustments, mess with the contrast, resize. THEN do your sharpening.
- When you’re sharpening, view your images at either 100% or 50%. Other viewing sizes will trip you up because of anti-aliasing weirdness.
- Printers and monitors are based on different technology, so you’ll always see things differently in print than you will on the monitor. If you’re planning to print, the monitor will help get you into the right ballpark. You’ll still need to make a test print, though, before you set your final sharpness levels.
The Almighty Unsharp Mask
We’re going to teach you how to use the Unsharp Mask filter. There are lots of other ways to sharpen, but this one works just fine and it’s a good introduction to the concept. We’re also going to use Photoshop as our example, but plenty of image editing programs have features that work the same way.
Pick a digital image you want to edit. Make a copy of the background and edit on that layer. That way if you screw up it’s no big deal. In the Filter menu, go to Sharpen -> Unsharp Mask. A window will pop up with three different sliders.
What The Sliders Do
Amount: When you sharpen an image, Photoshop takes the edge between two colors and makes the light pixels lighter and the dark pixels darker. Amount determines how light the lighter pixels get, and how dark the darker pixels get.
If you set the amount too high, your picture will look grainy and overly contrasty, and you’ll actually lose some fine detail.
Radius: This determines the area that will be sharpened. A low radius means only the pixels right next to the edge will be sharpened. A high radius means a wider area will be sharpened.
Setting the radius too high will give you weird outlines or halos around your edges. Yech.
Threshold: Threshold determines how much contrast there needs to be between colors for them to be sharpened. A higher threshold means higher contrast areas will be sharpened, but low-contrast areas will not. Sharpening low-contrast areas (like a baby’s smooth skin) makes them look rough and speckly.
Setting the threshold too low will give you a grainy look on low-contrast areas, and will make noise stand out. Not so good.
OK, So What Do I Do?
In a nutshell, you want to set the radius first, then the amount, then the threshold. Here’s how:
Step 1: View the image at 100%. Set the radius between 1 and 3. Set the amount between 300 and 500. Set the threshold at 0.
This will look like crap. But you’re going to fix it in a minute, so don’t worry.
Slide the radius level up until you start to see nasty halos forming, then back it off a bit. It’s OK if it looks a little bit harsh at this point.
Step 2: Change the image view to 50%. Adjust the amount until it looks grainy and oversharpened, than back it down a little.
Since web images need a fairly high amount of sharpening (in the 300 to 500 range), our example here isn’t quite as dramatic as we’d like. We made the “after” image a little soft so you can see what’s going on at this stage.
Step 3: Move the threshold slider up until the low-contrast areas look smooth, but you can still see fine details.
This is a pretty subtle adjustment; we zoomed in and overdid it in the example so you can see the difference.
Can’t I Just Cheat?
Yeah, we’re okay with that. Photography Jam has a good set of starting points for different kinds of pictures. We liked their all-purpose and web settings, but there are lots more on their site.
All-purpose sharpening: amount=85, radius=1, threshold=4
Sharpening for the web: amount=400, radius=0.3, threshold=0
These really are starting points, though, and you’ll want to play around until it looks right to you.
Where Do I Go From Here?
Congratulations bucko! Now you know how to sharpen your pictures using Unsharp Mask. It’s a good solid tool that should serve you well for most of your sharpening needs.
Of course, the thing about photographers is that we all have our own idiosyncratic ways of doing things. Some folks like Unsharp Mask, and some folks like other methods.
If you get really serious about sharpening, you’ll probably want to learn about other ways to do it. Check out Photoshop guru Ron Bigelow’s website for tutorials on smart sharpening, high pass sharpening, sharpening masks and a host of other techniques.
Here’s a taste of the wide world that awaits you:
Smart Sharpening: Sharpen mid-tones, highlights and shadows separately.
High Pass Sharpening: Sharpen on a layer instead of the picture itself. Special filters sharpen edges while leaving smoother areas untouched.
Sharpening Masks: Sharpening for Photoshop Grandmasters. Involves lots of layers and masks, but produces professional quality results.