Photoshop
Filters and Special Effects
Sharpen Filters
Sharpening is simply bringing a blurry image into focus. In addition to the rather limited Sharpen tool, Photoshop has four filters that do the job. Only one of these, the Unsharp Mask, has options for setting the strength and range of sharpening. In this lecture, we'll discuss and try out all four filters with particular emphasis on the Unsharp Mask.
An image can be blurred for a number of reasons: an improperly focused photo or one taken with a low-quality camera, problems during scanning, or as a result of resizing an image in Photoshop. Resizing is often unavoidable and even if you use the highest quality interpolation method, image quality can still suffer.
Practice Exercise: Using the Sharpen Filters
We'll try out the four sharpen filters on the same image so you can appreciate the differences in the results they produce. We'll resize the image initially and try to sharpen the blurring that resulted from resampling.
- Open park.psd.
- Go to Image > Image Size. Make sure that Constrain Proportions is enabled and set the Pixel Dimensions to 400x281. Click OK to resize the image.
- Duplicate the Background layer four times and name the duplicates Sharpen, Sharpen More, Sharpen Edges, and Unsharp Mask. Hide all but the Sharpen layer.
Sharpen and Sharpen More Filters
Both of these filters increase the contrast between adjacent pixels throughout the image to achieve a sharpening effect. The Sharpen More filter is stronger than Sharpen.
- With the Sharpen layer as the active layer, go to Filter > Sharpen > Sharpen. The effect is applied immediately and has had a noticeable effect. It's certainly sharpened the image but the highlight areas are quite contrasty.
- Make the Sharpen More layer the active layer and apply the Sharpen More filter to it. Ouch. In this image, Sharpen More is too strong and results in too much contrast and an unnatural look.
Sharpen Edges Filter
This filter gives more natural results than the first two. It looks for areas of significant transitions between colours and sharpens those areas. Other areas are unaffected, maintaining a smooth appearance.
- Activate the Sharpen Edges layer and apply the Sharpen Edges filter to it.
- At this point, you might find it helpful to hide the Sharpen and Sharpen More layers and then toggle the Sharpen Edges layer on and off to compare the results to the original image on the Background layer. It's easier to see the differences if you zoom in to 200% magnification. While there aren't any really smooth areas in this image, you might be able to notice that the grassy area was less affected than other more detailed areas.
Unsharp Mask
This filter finds differences in pixel values that exceed a certain threshold. Contrast is increased and applied to the designated radius area.
Let's look at the Unsharp Mask dialog before we begin. There are three settings you can adjust in the dialog.
- Amount
- Affects the amount of contrast that is applied. This amount can vary greatly depending on the extent of correction needed.
- Radius
- Determines the number of pixels from the edge pixels that will be affected by sharpening. Values of 1 or 2 are recommended. Values less than 1 will only affect the edge pixels. Higher values will affect a wider band of pixels.
- Threshold
- The Threshold value determines how much difference there must be in the colour values of adjacent pixels before they will be considered as edge pixels. Values between 2 and 20 are reasonable starting points. A value of 0 will eliminate the threshold and all pixels in the image will be sharpened.
- Make the Unsharp Mask layer the active layer and go to Filter > Sharpen > Unsharp Mask to open the Unsharp Mask dialog.
- Enable the Preview check box so you can view the effect in your image.
- Try playing with the three settings until you come up with a result that you're satisfied with. Click the OK button to apply the effect.
- If you want to save your file, save it with another name. Otherwise, you can close it.
You can view our results on a separate page. For the Unsharp Mask version, we used the settings in the dialog example shown above. The Unsharp Mask is far superior to the other three Sharpen filters in that you can set the range and strength of its effect.
Sharpen Filters Summary
- The Sharpen filter increases the contrast between adjacent pixels throughout the image.
- The Sharpen More filter operates the same way as the Sharpen filter but it applies greater contrast.
- The Sharpen Edges filter looks at areas containing pixels of sufficient contrast to be considered edges. The contrast of the edge pixels is increased without affecting other areas in the image.
- The Unsharp Mask filter has three settings you can adjust to customize the sharpening effect: Amount, Radius and Threshold. Because it's adjustable to suit the individual requirements of each image, this method of sharpening is the preferred method.
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