Sharpening is the process of finding edges in an image and making one side of each edge very dark and the other side very bright. An edge is an area in an image where the tone changes abruptly. Edges that have been sharpened appear to stand out more.
You can sharpen your images at any time for artistic direction but you are recommended to also sharpen early and at the end of your graph. By sharpening early on, you can bring back detail that was lost when the photo was shot. The end sharpening pass compensates for the softening that often occurs when preparing your image for the web or print. You’ll learn about output sharpening in Chapter 8. Output.
When taking photos with your digital camera, the resulting image will always appear softer than the original scene. This is an inherent property of capturing light and affects all photographs, regardless of camera or lens. If your camera is recording JPEG or TIFF files, it might apply some sharpening automatically. If your camera is recording a raw file, it’s up to you to apply any necessary sharpening.
The node primarily used for sharpening is the Unsharp Mask node located under “Filter” on the Nodes Toolbar. The name “Unsharp Mask” might sound like it does the opposite of sharpening (blurring) but its name comes from the fact it creates a mask to restrict sharpening to only part of an image. If you were to sharpen without this mask, the image would end up covered in awful artifacts.
You should always set the Canvas to a zoom of 100% before adjusting your image’s sharpness. Otherwise, the sharpening effect will be inaccurate because at other zoom levels, the image itself is approximated using interpolation. While this interpolation makes it convenient to work with images much larger than your computer’s display, it has the side effect of making your image appear softer or sharper than it actually is.
The Unsharp Mask node has three fields for controlling the sharpening effect. The idea is to balance these so that edges stand out just enough that they don’t overwhelm the subject.
- This controls the intensity of the edges that are sharpened. You’ll usually want to set this between 0.5 and 2.0 for most images. Images with smooth subjects, like people, will generally require lower values and images with lots of edges, like landscapes, higher values.
- How wide the edges are once sharpened. This setting varies wildly by the image’s size and subject.
- Controls how much difference in tone a pixel must be from its neighbors in order to be considered an edge to be sharpened.
The easiest way to find the right values for these fields is to set Amount to 10.0 and then adjust the Radius and Threshold sliders. Use the Radius slider to thicken the edges and the Threshold slider to suppress the noise. Finish by lowering the Amount slider so that the edges just barely stand out. Try not to obsess over the noise too much. A small amount of noise gives your image texture and is not usually as noticeable after output.
Another way to make it easier to see which areas you are sharpening is to connect a Greyscale node after the Unsharp Mask node. This is because it’s easier for your eyes to spot changes in grey than in color. You can always remove the Greyscale node once you’re done sharpening.