Sharpening

Why is sharpening necessary?

Scanning film, some digital capture and resizing an image all introduce a degree of softness into the pixels. When an image is digitized, the continuous tones are converted to pixels. Detail which is finer than the frequency grid used in the conversion is averaged so the final image appears softened.

Some printing processes will also soften an image and this must be anticipated.
Press halftones and inkjet printers print the pixels as dots of ink which softens the appearance.

The process to sharpen an image serves to increase the contrast between pixels. Pixels that are nearly white become pure white, while pixels which are nearly black become pure black. From this it is clear that if the process is over-cooked a little, the contrast becomes too great, detail is lost and unpleasant highlights, known as artifacts, appear.


Sharpening usually needs to be undertaken in two stages:

1. The first sharpening is to improve the original digital capture or scan. Digital cameras and scanners soften images to different amounts, so the image will require a little sharpening.

2. The second sharpening should be the final action on the file. Too much sharpening in the early stage will make a successful second sharpening much more difficult.
The amount of sharpening required here varies and is dependent on the final output; that is, it depends on the printing process, paper and ink being used.

• An image is sharpened with the Unsharp Mask Filter.

  Open any image.
  Go to Filter > Sharpen > Unsharp Mask.
  The filter works by increasing the contrast between pixels and a good balance needs to be achieved.

• Amount controls how much the contrast of the pixels is increased - it increases the edge differences. It works in conjunction with the Radius.

• Radius controls the width of the edge rims around the pixels. A higher value affects a wider band of pixels. The intention is to widen the edge rims to help the sharpening effect without producing a visible halo, as seen in Example 2 below.

• Threshold determines how much difference there must be between pixels before sharpening is applied.
  A Threshold of 0 will let all pixels be sharpened, which means there will be sharpening even in plain areas, resulting in unwanted noise. This will degrade areas of light or soft texture, such as skin.
  With a higher Threshold setting, these open areas will not be sharpened and the sharpening will only be applied to areas of detail.

• All settings are resolution-dependent, so a high resolution image will require higher settings. The settings used also depend on the image subject matter and on the type of output or printing process to be used.
  
  

• As a rough guide, start with Amount set at 200-300% and Threshold set at 0.

• Adjust the Radius.

  * Images with fine details should be given only a low Radius setting.
  * Images with areas of soft detail will require a higher Radius.
  * High resolution images will also require a higher Radius than low resolution images.

• Adjust the Amount again until a good balance is obtained. A small Radius will require a higher Amount.

• Finally adjust the Threshold to reduce noise and smooth out any soft textured areas such as skin or sky that have been over sharpened.

Example 1

Example 2

This image has less fine detail and more areas of little texture.
It has benefited from:

* a lower Amount of 60;
* a higher Radius of 2.2 and
* a higher Threshold of 10

• Amount and Radius interact with each other: if one is reduced, the other can be increased.

• Threshold determines how close tonal values must be for the filter to be effective - whether an edge is recognized. A higher number will exclude areas of lower contrast.
  If the Threshold is too low on a face, for example,
  it will look speckled and 'noisy'.

An example of sharpening gone mad. With too much sharpening, diffuse highlights have turned into pure white, creating unwanted highlight halos.
There is harshness and a noisy sky.

This is the basic way to use the Unsharp Mask filter. Used unsympathetically, the results can be crude. To avoid unwanted highlights and shadows and excessive noise in clear areas, there are better methods than the simple application of the filter.

• There needs to be enough sharpening to an images's edge contrast and definition without creating sharpening halos around edges, which are illustrated in Example 2.

• To judge the extent of sharpening, view the image at 100% by double-clicking on the Zoom Tool. A lot of anti-alias is used when an image is displayed at a lower percentage, which can give a false impression.

• Have the image set at its final output resolution and size before sharpening.

• If the output is going to be for halftone printing as in press or for dithered printing such as inkjet, sharpen so that the image looks a little over-sharp on the screen.

• If the final output is going to be for a continuous tone printer such as film recorders or Durst Lambda, sharpen so that the image looks good on the screen.

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