How Does Image Sharpening Work?

How Does Image Sharpening Work?

Have you ever stopped to wonder just how image sharpening works? One minute your photo is soft, but by moving around a few sliders it seems to snap into focus. How can that happen?

Even if you haven’t thought about this, there’s a good reason you should understand how image sharpening works. It allows you to make better use of the controls in tools like the strangely named Photoshop Unsharp Mask filter.

In this tutorial the example images have been sharpened using the Photoshop Unsharp Mask. It also refers to the Photoshop controls because these, or similar, appear in many sharpening tools.

How Image Sharpening Works

Sharpening works by enhancing the edges in an image to make them appear clearer. Photoshop does this by comparing adjacent pixels in an image to determine their difference in brightness. If it finds enough of a difference it detects there’s an edge to be sharpened.

Let’s look at a simple example using an image created from two graduated strips. The gradients in the strips run in opposite directions. Then when they are placed next to each other, we see an edge between the two.

Notice that you can clearly see the edge between the two gradients at either end of the strip. That’s because there’s a large difference in the brightness of adjacent pixels. At the extreme, one pixel is white whilst the other is black.

Now compare this to the area in the centre. Notice the separation between the two strips is more difficult to see. That’s because there isn’t a large difference in the brightness of the pixels. In the very centre of the strip, the pixels will have the same level of brightness so there’s no difference.

Controlling Image Sharpening

When tools like Photoshop identify an edge, they sharpen that edge. This is done by increasing the contrast between adjacent pixels. This means dark pixels become darker and light pixels become lighter.

If you’re using a sharpening filter that has a strength or amount slider, it’s this contrast level that its controlling. As you increase the strength of the sharpening, you are increasing the contrast difference between the adjacent pixels. You can see this in the following example where sharpening has been applied to our strips.

Here we’ve applied image sharpening which makes the edge between the two strips clearer and easy to see. If you look closely in the highlighted areas, you can see a faint line along the edge. This edge is called a Sharpening Halo and is where the contrast enhancement is applied.

It’s usually possible to control the width of this edge using controls like a Radius adjustment. This controls how wide the image sharpening halo is and therefore how easy it is to see. When there is a wide halo, as in the following example, it becomes obvious.

It’s usually best to avoid creating obvious image sharpening halos like this as they become distracting. They can also damage the appearance of fine detail in an image because they make thicken edges. How much of a problem this becomes will depend on how far you are from the image you are viewing. An image sharpening halo that’s 10 pixels wide may look terrible if the image is a few inches from your eye. But when it’s 20 feet away, you probably won’t notice it.

Threshold Edge Detection

One final adjustment that’s common when using image sharpening filters is something called Threshold. This controls how sensitive the software is when detecting edges. If we use a low threshold, we don’t need a large difference between adjacent pixels to detect it as an edge. But if we increase the threshold, there needs to be a greater difference between the pixels for it to be identified as an edge.

Here’s an example where the Threshold is set to a large value.

Notice how we can still see the contrast edge towards either end of the strips. But then in the central area there’s no image sharpening. That’s because the pixels in this area are too similar, which is being controlled by the high Threshold value.

Now compare this to an example with the same setting but using a small Threshold.

Notice the difference between the two examples, even though it’s only the Threshold setting that’s been changed. Not only does the image sharpening halo appear thicker, but it’s seen all the way to the centre of the strip. A Threshold control therefore provides a way to avoid sharpening unwanted detail and focus our enhancement on true edges.

Summary of How Image Sharpening Works

Whilst this article has mentioned several controls from the Unsharp Mask filter in Photoshop (Strength, Radius, Threshold) it’s likely you will find similar controls in other sharpening tools. Image sharpening works by enhancing the contrast along edges to make them clearer and easier to see. As a result, the image appears sharper. What the controls do is allow you to adjust the strength and width of the sharpening enhancement as well as where it’s applied.

More recently, a new breed of sharpening tool has arrived which uses AI technology to sharpen images and remove blur. One of the best that I’ve used is Topaz Sharpen AI.