Basic sharpening

by Reg Caldecott

At almost all judging sessions at the club mention is made of “sharpening” or to be more precise, “over sharpening”. I think it would be extremely difficult, make that …impossible, to try to attach concrete values as to how much an image should be sharpened and at what limits an image would be considered over-sharpened. There are just too many variables as well as different methods that can be used to sharpen photographs.

As a first step I think we need to understand a little of the background of some of the methods of sharpening and how to apply them. The most common method, according to Google, is using the “unsharp mask”. Not wanting to re-invent the wheel and more importantly not being able to write it better myself the following article is used verbatim with acknowledgement to www.

The biggest problem I have with this method is that the entire image (Including the “grain”) is sharpened and this often results in parts the picture appearing “noisy”. This can be countered by applying a layer mask or using high pass sharpening, but more about that in Part 2.

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.

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 colour 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 colour, so it renders it as grey.

What we think of as sharpness is actually the contrast we see between different colours: a quick transition from black to white looks sharp. A gradual transition from black to grey to white looks blurry. So when we look at the picture you just took of that sharp black & white edge, the grey pixels along the edge will make the photo look blurry.

Sharpening your picture increases the contrast along the edges where different colours 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.

  1. 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.
  2. Don’t sharpen until the last step of the editing process. Crop, make all your colour adjustments, mess with the contrast, resize. THEN do your sharpening.
  3. When you’re sharpening, view your images at either 100% or 50%. Other viewing sizes will trip you up because of anti-aliasing weirdness.
  4. 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 colours 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 contrast 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 colours 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 speckled.

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 over sharpened, than back it down a little.

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.

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.

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.

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