Sharpening Your Image
Time to look at the sharpening functions. Like any of the functions that control color and detail, there is a way to do it and a way not to do it. Too often, people just hit the "Sharpen" and let it be. Well, that is one way to do it, but it surely is not the best.
First of all, let me mention the options. In Photoshop, you have these choices: Sharpen, Sharpen Edges, Sharpen More, and Unsharp Mask. In Paint Shop Pro, you have these: Sharpen, Sharpen More, and Unsharp Mask. It would be a waste of time to explain most of these. Trust me on this one, the only one you need is the Unsharp Mask in either program, or any program for that matter. The others are merely automated applications of the Unsharp Mask.
If you have been reading these pages, you must have come to the conclusion that I don't trust computers, and I don't. At least, I don't trust them to make decisions about how to apply filters. In the sections about brightness and contrast, you saw how these automatic filters can damage your image. Sharpening your image is no different. I don't have a tool like the histogram to show you this, but the automatic sharpening filters can damage your image just as severely. The problem is that the computer can not see the image, and it can not distinguish detail from noise. When you are doing delicate work, you want to use a razor, not a battle axe. When sharpening your image you have to examine the image and apply the filter where it is needed.
All of these functions except the Unsharp mask apply a fixed setting over the entire image. They sharpen noise as well as detail. Why would you want to do that? Even the edge filter does this. It can't tell that an edge is a flaw in the print or not, and will sharpen that edge just like any other. The Unsharp mask allows you to selectively sharpen what needs it, and ignore the rest.
Of course, the problem now becomes how to identify the detail you want. That is what you will learn here. But first, let's see how the Unsharp Mask works.
The Unsharp Mask
The Unsharp masks by identifying edges and then increasing the contrast between neighboring pixels on the edge. It does this by darkening the pixels on the darker side of the edge and lightening the pixels on the lighter side. You see this as a more sharply defined edge, and so the image looks sharper, clearer, and more in focus. Here are the dialog boxes for Paint Shop Pro and Photoshop.
Paint Shop Pro | Photoshop
These two boxes are about the same, only the terms they use are different. Radius is the same in both boxes. The radius is the thickness of the line of contrast the filter will place on the edge. The higher you make this, the thicker the edges will look. The amount/strength, obviously, is how much the filter will effect the image. The clipping/threshold sets a limit on the filter. It tells the filter this. If the difference between neighboring pixels is this much or less, you will ignore them. The auto proof/preview show you what the effect will look like on your image.
Now, In order to use this well, you have to think about your image. Suppose what you have is a green plant on a red table on a blue carpet. If you look at the red channel of this image, the lightest area in the red channel will be the table, in the green channel it will be the plant, and in the blue it will be the carpet. The Unsharp mask will work on the luminosity values, so in each channel, it will put the light and dark halo in a different area. At the table's edge with the carpet, you will get a light line on the table side and a dark one on the carpet side in the red channel. You will have the opposite in the blue channel. Now you have a bright red line over a dark blue line. Good formula for a purple shift. You wouldn't notice this too much normally, but as you increase the radius, it becomes more noticeable. A color shift at the edges is a definite possibility here.
Also consider this. If you examine the individual channels, you will find that some have more detail than others, and some, especially the blue and yellow, will have more noise. You wouldn't want to sharpen noise, and there might be some detail you want to sharpen and some you might not want to sharpen. For instance, you will want to sharpen edges, but you don't want to sharpen every pore and wrinkle on a person's face.
If you agree with these points, you will see that applying this filter over the whole image can be costly to your image. You would avoid this by being selective about what you sharpen. Here are two ways to do this.
First thing. The Unsharp mask works by affecting the luminosity values of adjacent pixels. It would make sense to work in a channel that just has luminosity values, then, wouldn't it? And indeed there is a channel available that does just this. In Photoshop, there is a color mode called LAB, and guess what the L in Lab stands for? And in Paint Shop Pro, there is a color mode called HSB, And the B stands for Brightness (Luminosity). If you convert to these modes first, and then just sharpen the L or B channel, you can enhance your image without any color shift.
This is fine if you have a good clean image, or if there is no detail you want to avoid. If you have noise, though, some of it may show up in these channels. Noise has luminosity, too. In this case, what you have to do is examine each channel separately, and work on the channels that have the detail you want, avoiding those with noise and unwanted detail. You might still get a color shift this way, but usually that would be less objectionable than enhanced noise would be, and you might be able to correct the color shift.
Make sure you examine ALL the color channels. Don't think in a box. There are at least ten separate channels available in each of these programs.
Available Color Channels
Photoshop
Red
Blue
Green
Cyan
Magenta
Yellow
Black
Luminosity
A (LAB)
B (LAB)
Paint Shop Pro
Red
Blue
Green
Cyan
Magenta
Yellow
Black
Hue
Saturation
Brightness
You wouldn't normally sharpen the A or B channels in LAB, but those channels are good for removing color noise. You wouldn't want to sharpen the Saturation or Brightness channels, either. But this leaves you with eight channels that could be looked at. So make sure to look at all the channels. You may find that you can enhance your image by slightly blurring the channels with noise and sharpening the ones without noise.
How To Use The Unsharp Mask
This filter has three settings, and deciding how to set them is tricky. Here is a way to decide how to set them for your image. When you first call up the Unsharp Mask, set the strength/amount to 500. This will make your image look horrible, but it will let you easily see what is being affected. Set the radius and the clipping/threshold to 1. What you want to determine first here is the threshold. Gradually increase the threshold until you see that the only detail being affected is the detail you want. Then, set the radius to the value that looks the best to you. Finally, decrease the amount of sharpening until you have what you want. I would say that my usual settings are about threshold 10, amount 125, and radius 2. These values, though, are related somewhat to your image size, so you have to play with them, and you will find a wide variance in what you will need depending on your image.
Sharpening The Smart Way
1. Sharpen in the L (LAB) channel or the B (Brightness) channel as much as possible.
| This will minimize color shifts that the unsharp mask can create.
2. Try to avoid channels that are showing a lot of noise.
| When sharpening in other channels, sharpening just the channels that are clean will achieve the desired effect without increasing noise in most cases.
3. If noise is a problem, consider using an edge mask.
| In the next section, we will be talking about masking. A mask is an image you create to control the effect of your filters. There is a procedure there for creating and using an edge mask.
4. Sharpen on a duplicate layer
| Duplicate the layer you want to sharpen, and do your sharpening there. This allows you to reduce the opacity of the layer if you sharpen too much, it allows you to erase any areas you don't want to sharpen at all, and it can be converted into a Layer Mask, another kind of mask we will discuss. Remember, the easiest way to undo something you did is to delete the layer it is on.
A Quick and Dirty Sharpening Technique for Photoshop
Here is a quick sharpening technique using the High Pass filter. This is easy, but you will have to experiment with it. Take the image you want to sharpen, and duplicate the base layer. Apply the High Pass filter to this layer (Filter, Other, High Pass). The lower the value you use, the better this usually works, but like anything, you have to pick the value that looks best to you. I used a value of 3 for this image, and it looked like this. |
Original Image | With High Pass layer in Overlay mode
With High Pass layer in Hard Light mode | With High Pass layer in Soft Light mode
Now set the layer properties on the High Pass layer to Overlay, Hard Light, or Soft light. The neutral gray will not effect the image, but the effect of the High Pass filter will tend to accentuate edges, making the image appear sharper. Each of the modes (Overlay, Hard Light, Soft Light) will have a slightly different effect. I tend to like the Soft light look, because it seems less harsh to me. If you think the effect is too pronounced, all you have to do is reduce the opacity of the High Pass Layer until it suits you. Quick and easy.
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