SAN FRANCISCO (02/18/2000) - No matter how good your scanner or how crisp your original, you're certain to lose some sharpness when you digitize an image.
Images from low-end flatbed scanners always need a considerable amount of sharpening. Even scans from high-end scanners need sharpening (though scanning software sometimes takes care of it on the fly). Images from digital cameras can also benefit from sharpening.
An image loses sharpness not only in the scanning process but also in the output process. Halftones (almost anything on a printing press) and dithered graphics (such as those printed on ink-jet printers) are the worst offenders.
But even continuous-tone devices such as film recorders and dye-sublimation printers reduce sharpness.
Remember: You cannot solve the problem of blurry scans by scanning at a higher resolution. And while there's no way to bring an out-of-focus original back into focus, even a blurred original (such as a fast-moving subject) needs sharpening so it won't be blurred and soft.
To counteract the blurries in both the input and output stages, you need to sharpen your images. When it comes to sharpening, the key tool is Adobe Systems Inc.'s Photoshop's Unsharp Mask filter, the master at making your images crisper (despite its counterintuitive name).
The Unsharp Mask Filter
The reason you can see these printed words so clearly is that they have well-defined edges. Our eyes are extremely good at discerning edges-sharp lines between one color and another. The more marked the difference between two colors along an edge, the sharper an image and the faster we understand what we're looking at.
The Unsharp Mask filter (Filter: Sharpen) compares each pixel in your image to its neighboring pixels; the greater the contrast between two pixels, the more the program increases the contrast. This results in a halo effect around edges that appears to increase an image's sharpness.
The Unsharp Mask filter can also have the undesirable effect of exaggerating texture in flat areas, noise introduced by a scanner, and artifacts from JPEG compression. Fortunately, the filter offers some controls to help you walk the fine line between sharp and ugly. The trick is to strike the right balance using the Amount, Radius, and Threshold controls in the Unsharp Mask dialog box.
The Whens and Hows
When sharpening, you need to think about restoring sharpness lost in the image-acquisition process and introducing extra sharpness to compensate for its loss during the output process. For the highest-quality image, you need to know where your image is going before you sharpen it. Sharpening an image destined for newsprint is a different game from sharpening for the Web. Printed images, especially those at low halftone-screen frequencies (such as newsprint images), require more sharpening because subtle edge halos get lost in the translation to halftone dots. On-screen images, however, often need just enough sharpening to offset blurriness acquired at scan time.
Note that you should usually use the Unsharp Mask filter near the end of the image-correction process (after color correction), but before you convert from RGB to CMYK or set your minimum highlight dots. If you sharpen afterward, the Unsharp Mask filter might create unwanted specular highlights-near-white pixels blown out to pure white.
Also, if you use the Image Size dialog box to reduce the size of your image by more than about ten percent, you'll probably need to run the Unsharp Mask filter again. That's because downsampling softens the contrast between pixels.
Look Sharp
The Unsharp Mask filter is a powerful tool. Used well, it can give your images an extra snap that makes them jump off the page. Used badly, it can produce an unpleasant high-contrast look. Applying the Unsharp Mask filter is definitely a skill that improves with experience. The more you experiment with the controls, the closer you'll bring your image to perfection.
DAVID BLATNER is a coauthor of Real World Photoshop 5 (Peachpit Press, 1999), including the 5.5 update at http://www.peachpit.com/photoshop/.
Sharpening What You Want
Sure, I said every image needs sharpening, but that doesn't mean you want to sharpen every pixel in an image. If you examine the individual channels of most RGB scans, you'll find that often the blue channel is by far the noisiest of the three. It's also usually the one with the least-important details, so in most cases you can get away with sharpening only the red and green channels and perhaps using the Despeckle filter on the blue channel to even it out. This works best with lower Radius settings (1.4 and under). Some people convert the image to Lab mode and then sharpen the Lightness channel, even though the mode changes can cause minor degradation.
Another way to sharpen part of an image is to do it through a mask, as shown in the example here. For instance, you may want a little additional sharpening in and around the eyes in a portrait, or you might want to avoid sharpening a picture's background in order to focus attention on a foreground object.
After duplicating the background layer and selecting what you do want to sharpen, click on the Add Layer Mask button in the Layers palette. Feel free to touch up this mask with black and white paint to fine-tune the selection.
Run the Unsharp Mask filter on the duplicate layer (the image, not the mask).
Only the part that is not masked appears sharpened. If the mask's edge is too hard, the transition between sharpened and unsharpened pixels will be too obvious; you can use the Gaussian Blur filter on the layer mask to soften this transition.
To create a more severe contrast between foreground and background, I selected the background layer and softened it with the Gaussian Blur filter.
The Simple Art of Sharpening
Many people get nervous when confronted with the Unsharp Mask filter for the first time. Don't worry-it's a simple three-step process. (But make sure to leave yourself an escape route by either working on a duplicate of your image or archiving the unsharpened version first.)1.Open the Unsharp Mask filter's dialog box (Filter: Sharpen). I like to start with a significant change, so I set the Amount control (the strength of the sharpening effect) to 200 percent and the Radius control (the size of the sharpening halo) to my image's resolution divided by 200. For instance, with a 300-pixels-per-inch image, I'd start with a Radius setting of 1.5 (300 divided by 200). I set the last control Threshold to 4, which in effect tells Photoshop to ignore pixels that are less than 4 levels apart on the tonal scale.
If you select Preview mode, Photoshop shows you what your image is going to look like after you click on OK. It's important to view your image at 100 percent (Actual Pixels), since that's the most accurate view. The good news is, you don't have to close the dialog box to do so, because you can zoom in and out from within the dialog box.
2.Next, fine-tune the image. If it has a lot of detail (for example, a picture of bare tree limbs against the sky), you need to lower the Radius setting and increase the Amount setting. If it's an image with large objects that have slow color transitions - say, a portrait or a picture of a field of grass - increasing Radius and decreasing Amount will probably improve your image the most. These two settings usually work like a seesaw - as you adjust one up, you should send the other down.
The Unsharp Mask filter accentuates noise - whether it's from JPEG artifacts, your scanner, or a dirty original. You can control this to some degree by increasing the Threshold value. However, if you're using Thresholds higher than 8 or 10, you should probably start looking at other methods of controlling noise (see "Sharpening What You Want").
3.Finally, feel free to throw all these rules out the window and use the settings that work best for your image. Don't be afraid to use a Radius setting as small as 0.3 or so, especially if you're producing images for the Web.
Making the image sharp enough on your screen will usually translate to a sharp image on the viewer's screen.
If you will be printing the image, in general the lower the halftone-screen frequency (or the lower the printer resolution), the larger you should make the radius. (If your halo is much smaller than your halftone dot, it will simply disappear, and so will your sharpening.) Note that there's really no way to get an accurate on-screen preview of how your sharpened image will look in print (the screen image is simply too different from the halftone output).
However, an image that's well sharpened for halftone output often appears decidedly oversharpened on a monitor. If you have doubts, it's better to err on the side of cautionÑmost viewers find a soft image less disturbing than one that's too sharp.
Sharp Tip: Make a big change, then fine-tune it: change Amount to 200, then adjust up or down until the image looks right.
Behind the Unsharp Mask Filter
The Unsharp Mask filter can't actually make an image sharper-it just makes the image appear sharper by intelligently enhancing the contrast between pixels.
This filter accentuates the transition between light and dark pixels at edges: the dark side becomes darker and the light side becomes lighter, creating a halo effect around an object's edges. Because Photoshop has to look at every pixel, applying the Unsharp Mask filter can take a long time, especially with large images.