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Photography

What Is Image Sharpening?

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This post is part of a series called How to Sharpen Digital Images.
The 7 Hidden Dangers of Image Sharpening

Sharpening: every photographer knows they should be doing it to their images but very few understand how it works and why it’s important. In this series of tutorials we're going to explore sharpening in depth. Rather than just parrot half-truths and poorly understood technical advice, I’ll lay out everything you need to know. At times this might get technical, but trust me, it’ll be worth it. In this tutorial you will learn what sharpening actually is.

Ready? Let’s get started.

What is Image Sharpness?

Before getting into the act of sharpening an image, we need to consider what sharpness actually is. The biggest problem is that, in large part, sharpness is subjective. 

Sharpness is a combination of two factors: resolution and acutance. Resolution is straightforward and not subjective. It's just the size, in pixels, of the image file. All other factors equal, the higher the resolution of the image—the more pixels it has—the sharper it can be. Acutance is a little more complicated. It’s a subjective measure of the contrast at an edge. There’s no unit for acutance—you either think an edge has contrast or think it doesn’t. Edges that have more contrast appear to have a more defined edge to the human visual system.

t test
Both Ts have the same resolution but different acutance. Which one looks sharper?

Sharpness comes down to how defined the details in an image are—especially the small details. For example, if a subject’s eyelashes are an indistinct black blur they won’t appear sharp. If, on the other hand, you can pick out each one then most people will consider the image sharp. 

What is Sharpening?

Sharpening then, is a technique for increasing the apparent sharpness of an image. Once an image is captured, Photoshop can’t magically any more details: the actual resolution remains fixed. Yes, you can increase the file’s size but the algorithms any image editor uses to do so will decrease the sharpness of the details. 

In other words, the only way to increase apparent sharpness is by increasing acutance. If you want your image to look sharper, you need to add edge contrast.

Unsharp mask in action
Here I've mimicked the Unsharp Mask effect in Photoshop. The first image is the original file, the second is a blurred copy, the third is the one subtracted from the other so as to detect the edges, and the fourth is the original image sharpened using the edge layer. Unsharp masking is the oldest sharpening technique. It subtracts a blurred (unsharp) copy from the original image to detect any edges. A mask is made with this edge detail. Contrast is then increased at the edges and the effect is applied to the original image. While unsharp masking was originally a film technique, it’s now the basis of digital sharpening.

Why Sharpen Images?

There are three main reasons to sharpen your image: to overcome blurring introduced by camera equipment, to draw attention to certain areas and to increase legibility.

RAW files from any modern camera are always slightly unsharp. Every step of the image capturing process introduces blur. As the light passes through the lens elements—no matter how well made—some definition is lost. When the sensor processes the photons falling on it, the sharpest transitions are averaged out and slightly blurred. When the three different colour channels are interpolated to create the final image, again, a small amount of blur is introduced. 

straight out of camera
A straight out of camera file versus a sharpened one. I've removed all colour toning and retouching for the best comparison.

Second, human eyes are attracted to contrast. When we look at a photo, we are drawn to the sharpest details. If you’re trying to direct a viewer, selective sharpening is one of the best ways to do it.

Finally, sharpening an image makes it easier to see important details. Text becomes easier to read, individual leaves stand out and faces in a crowd become more distinct. 

Basic Sharpening in Adobe Photoshop

While I’ll dive into more detail in a later tutorial, let’s have a quick look at how to sharpen an image in Photoshop using the Unsharp Mask filter. This will give you an understanding of how image editors increase acutance. 

Open a test image in Photoshop and create a copy of the Background layer. Go to Filter > Sharpen > Unsharp Mask. This will bring up the dialogue box you see below.

unsharp mask filter
Using the Unsharp Mask filter.

There are three sliders for you to play with: Amount, Radius and Threshold

  • Amount is a percentage value between 1 and 500. It’s a measure of how strongly the edge contrast effect is applied. 
  • Radius controls the blur amount of the unsharp layer. You can blur the copy by anywhere between 0.1 and 1000 pixels. The less blur you apply, the finer the edges that will be detected. 
  • Threshold is used to prevent over-sharpening. Even in an area of continuous colour, there will be small amounts of variation. Threshold determines the minimum brightness variance before edge contrast is added.

Enter an amount value somewhere between 100% and 200% based on how strongly you want to sharpen your image. Drop the radius to 0.1 then slowly increase it until edge contrast is added to the details you want to emphasise. Finally, look at any flat areas of your image, especially shadow areas. Increase the threshold until any noise that’s been accidentally sharpened is gone. That’s the very basics of sharpening a digital image.

Sharpening Considerations

When you’re sharpening, there’s a lot of factors you need to take into account. The resolution of the file, the final medium it’ll be displayed on and size of the details in the image itself are all important considerations.

One of the biggest mistakes photographers make when it comes to sharpening is applying it to a full resolution image and then thinking they’re done. Different levels of sharpening are required for different final image resolutions.

Sharpening should be done at the absolute end of the post-production process, once everything else has been done, especially resizing. For example, a high-res file for printing requires relatively less sharpening than a low-res web export for the effect to be noticeable. Whenever you are sharpening an image, you should convert it to the final export resolution before applying it.

good versus bad sharpening
The image on the left is appropriately sharpened for the resolution. The image on the right is badly over-sharpened.

The final medium an image will be displayed with also determines the amount of sharpening that’s required. Images that will be displayed on screens, such as images for your website, appear sharper—so they require less sharpening—than print images. For printed images, different materials and kinds of paper require different levels of sharpening. Images displayed on glossy paper need less sharpening than those printed on matte paper. Other surfaces like metal, acrylic or glass all determine how sharp a printed image appears and thus how much sharpening is needed in post production.

The size of the details you want to emphasise is one of the most important factors. If you’re printing a portrait, it’s unlikely you want every individual pore on the model’s skin to be visible. On the other hand, for certain landscapes, you might want every blade of grass to stand out. The details you want people to see should be sharpened and those you want them to ignore should not be. This may require more than one sharpening filter to achieve.

Continuing On

Over the next few tutorials in this series, I’m going to explore all aspects of sharpening.

First, we'll tackle the tricky problems that come with sharpening. We'll look at why not all images benefit from sharpening, and how local sharpening is a good choice for many pictures. You'll learn why too much contrast is a bad thing, and why the best sharpening is always found in the midtones.

Then you'll learn about different techniques for sharpening, and the kinds of images each is suited to. You'll learn how to apply sharpening the right way, and how to use sharpening to enhance your images without introducing unwanted artefacts.

Last, you'll learn some advanced sharpening techniques. Grain and noise have a bad reputation among photographers, but they're actually very useful for sharpening! You'll learn how texture can enhance resolution and hide defects, and why "perfect" digital images can need noise to be percieved properly as photographs.

To recap, sharpening a combination of two factors: resolution and acutance. Resolution is determined by your camera’s lenses and image sensor, and once you’ve taken the picture, there’s no way to add extra data. Acutance is a subjective measure of edge contrast: this is what you can increase to make your images appear sharper. Sharpening isn’t complicated, it’s just poorly understood, but with a little bit of knowledge and practice it can be a very powerful and nuanced tool.

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