Why is it necessary to sharpen digital files and how is it actually done? We explain all there is to know
The aim of sharpening is to make an image appear sharper. Cameras can apply a certain amount of sharpening automatically to JPEG files, but raw files should be sharpened post-capture using raw conversion or image-editing software prior to printing. Whichever file format you’re dealing with, though, sharpening works in the same way.
When a file is sharpened, the contrast between the pixels in the picture is increased, giving the appearance of a crisper, more detailed image.
Why do digital images need sharpening?
Images need sharpening because of the way in which they are created. Unlike film, where the particles of light-sensitive chemicals are packed tightly together in layers, the pixels on a digital camera or scanner’s sensor are arranged in a grid, with regular spaces between them. This creates the ‘softness’.
In digitally created images, the sensor design can result in distortion artefacts, and patterning appears when the sensor cannot resolve detail beyond a certain frequency. In order to reduce this effect, almost all digital cameras are fitted with a very thin anti-aliasing filter, which is positioned just in front of the sensor. These filters (which also usually block infrared light) ‘downsample’ (or blur) the pattern arriving on the sensor, and can slightly reduce overall sharpness, resulting in an ‘unsharp’ image.
Unsharp vs out of focus
An unsharp image is not the same as an image that is out of focus. Sharpening cannot ‘correct’ a focusing error, because the cause of the problem lies in how the photograph was taken, not how it was recorded and processed. An out-of-focus image will always be out of focus. Look at the out-of-focus image below and note how sharpening increases the overall contrast, but cannot recover any real impression of detail – there isn’t any.
Edge sharpening compared to detail sharpening
Every photographer has a preferred method of sharpening, which may differ depending on the type of image being worked on. However, there are two main types. Edge sharpening simply increases the contrast between areas of contrasting tone, which increases the appearance of sharpness around edges. Detail sharpening performs this action universally, on the entire image area.
In images that contain a lot of detail, like the stonework in the picture below, it’s a good idea to apply a general Unsharp Mask to increase the definition across the entire image area.
You’ll be amazed by the difference that sharpening can make to a shot, and by the amount of detail that suddenly seems to appear once sharpening is applied.
One of the areas in which ‘unsharpness’ is most noticeable is where an area consisting primarily of one tone borders another area of a significantly different tone.
By sharpening this edge, the border between the two elements of the scene is made sharper, and the image appears more three-dimensional as a result.
The photograph of the airliner below demonstrates this effect, but for an extreme example a graphic representation below shows an edge between two ‘flat’ tones, black and white. In the upper of these images, there is a thin line of greyish pixels between the black and white areas.
In a real image this would represent values recorded by photosites on the camera’s sensor that straddle the two tones. The pixels are grey because the camera’s processor does not know if they are meant to be black or white.
Most image-manipulation programs feature simple ‘on/off’-type controls for sharpening, but many also offer an Unsharp Mask, too.
Often abbreviated to USM, Unsharp Mask provides compete control over the amount of sharpening applied to an image.
We’re looking at the Photoshop USM dialog here, but most USM tools operate in the same way, with the same main controls.
Think of the Amount slider as the volume control of Unsharp Mask. Sliding it to the right increases the effect of the sharpening, while dragging it to the left decreases it. The level at which this slider should be set depends on the type of image, as well as the Radius of sharpening used. A large Amount with a low Radius and low Threshold is good for general detail-sharpening.
The Radius (in pixels) is controlled using the second slider, and alters the size of the area that is ‘pulled apart’ to form a sharpened edge. Generally, the larger the Radius set, the lower the Amount required. The relationship is not exactly proportional, but a large Radius coupled with a high Amount may result in an unwelcome contrast increase, and haloing.
The final slider, Threshold, controls exactly which elements of the image will be sharpened. Setting a high Threshold tells the software to look for obvious edges, whereas setting a low Threshold will sharpen everything. For this reason, a low Unsharp Mask Threshold is better suited to images where a lot of fine detail needs to be extracted, such as a landscape or city scene, and a high Threshold is great for edge sharpening.