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November
2004

Changing the Focus: Sharpening & Blurring in Photoshop

Fig. 1
Two very flexible tools in Adobe Photoshop, Photoshop Elements and other image manipulation/paint programs are the sharpening and blur tools. These tools allow you to help imperfect images or create dramatic effects. This month, I’ll look at each one.

You will find these tools in two places: under the Filter menu and in the Tools palette (Fig. 1). The filter versions will apply the effect globally to the entire image or to the active selection area; the tool versions allow you to paint with a selective sharpening or blurring effect.

Photoshop sharpens an image by analyzing the relative darkness and lightness of neighboring image pixels. Most digital images, whether from a scanner or a digital camera will benefit from some degree of sharpening. Photoshop’s sharpening tools will help enhance a good image, but will not work to sharpen up an overly blurred image. This example shows that even using very high setting with the Unsharp Mask filter will not significantly improve an out-of-focus image (Fig. 2).

Rarely do I actually use the Sharpen or Blur tools from the Tools palette. These tools tend to provide results that look incongruous with the rest of the image. If I want to selectively sharpen or blur an image, I create a mask with the selection tools.

Fig. 2

Let’s now focus on the Sharpening tools available under the Filter menu: Sharpen, Sharpen Edges, Sharpen More and Unsharp Mask (Fig. 3). All four of these are available in both Photoshop and Photoshop Elements. The first three options give you no actual control over the amount of sharpening of your image. Different images will require different amounts of sharpening. A high-resolution will need a higher level of sharpening to achieve the same effect that you would get with less sharpening on a Web-resolution image. You may also not want to sharpen an image that is exceptionally grainy as much as you would a smoother image. The sharpening would over-accentuate the grain of the image. For all of these reasons, most professionals use the Unsharp Mask filter exclusively. This filter increases the perceived sharpness by increasing the contrast of the edges found in the image. It gets it rather unintuitive name from an old traditional darkroom technique that used a slightly out-of-focus negative sandwiched with a good negative to obtain better edge detail.

Fig. 3

The Unsharp Mask filter has three settings. The first, Amount, controls how strongly the sharpening effect is applied. For print images an Amount between 150 and 200% is usually good, although you can go higher if you feel you need to. Sharpening is often a judgment call. Look at the image at the approximate size it will be used when previewing the results. The next slider controls the Radius of the effect. This is how many pixels out from an edge will be affected. Too large a Radius value will produce an obvious and unflattering halo effect. The last setting is the Threshold. This is the tonal value difference of pixels that will be affected on a scale of 0 to 255. A Threshold of 0 will affect all pixels. A value of 4 will not affect adjacent pixels that have a value of, say, 199 and 201. Threshold should usually be set fairly low, between 2 and 10. More than that will diminish the effectiveness of the filter too much.

Now we will move to the Blur filters, of which there are more options: Average, Blur, Blur More, Gaussian Blur, Lens Blur, Motion Blur, Radial Blur and Smart Blur (Fig. 4). Average and Lens Blur are new to Photoshop CS, the others are available in Elements 2 as well as Photoshop 7. Again, the pros have their favorites among the Blur tools. The Blur and Blur More filters give no controls over the amount of the effect, so I usually use the Gaussian Blur tool. This is a pretty easy one to comprehend since it has only one setting: Radius. This is the amount, in pixels, that Photoshop averages together to make a soft focus look.

Fig. 4

The other filters give some varied results. The Average filter will replace the image of active selection area with a flat color that is the average of all the selected pixels. Applied on a cloudy blue sky, you will get a flat field of light blue.

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Lens Blur is a tool aimed at photographers to help simulate the selective focus you get with different camera lenses and varied depths-of-field. It tends to work best if you have a mask separating the parts of the image that are at different depths.

Motion Blur gives you the sort of effect you see when you photograph an object in motion. You can select both the distance and the angle of the effect.

Radial Blur allows you to simulate the blur effect you would get by rotating or zooming the camera while shooting your picture. Here you can pick not only an Amount, but also the Blur Method you want to use (Spin or Zoom) and the Quality of the rendering (Draft, Good or Best). This is one of the most intensive functions in Photoshop, if you select a high amount and the high quality rendering, be prepared to step away for some lunch if you don’t have a very fast computer.

The Smart Blur filter attempts to give the same sort of control over blurring an image that Unsharp Mask does for sharpening. You can preserve the edges of the image while blurring the other areas. Smart Blur gives you sliders to control the Radius and Threshold of the effect as well as pop-up menus governing the Quality and Mode of the filter. The Normal Mode behaves as you would expect, Edge Only just shows the edges of the image and Edge Overlay draws the edge lines over the blurred image. Most Photoshop users will have little use for these last two modes.

Here is an example of a typical use of the Gaussian Blur filter. I have a charming photograph, but the background is a little busy. If I were a really good photographer with good equipment, I could have made sure that the objects in the background were out of focus. First I duplicated the image as a new layer (Layer > New > Layer from Background) and made a Layer Mask isolating the subject. Then I duplicated the background layer again, added a gradated Layer Mask and ran the Gaussian Blur filter at a fairly high Radius. Since the Layer Mask was white at the bottom and black at the top, the blur effect shows more at the top of the image. The subject of the photo remains sharp since he is on a layer above everything else (Fig. 5).

Fig. 5

The Blurring and Sharpening tools are crucial components in the Adobe Photoshop toolbox. Learning how they work gives you new, creative ways to enhance your images.

Paul Vaughn is a freelance graphic artist, writer and web designer. While I prefer to be sharp, I sometimes end up blurry! If you would like to see the Graphics Guy address a specific topic email Paul Vaughn at paulv@mac.com.

 

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