While sharpening has the desirable affect of increasing the contrast along edges, it can also have the undesirable affect of causing a color shift along the very edges that are being sharpened. Figure 1 (shown at 200% view) shows an example of color fringing. The sharpening in this example has been exaggerated to make it easier to see the color fringing. The power line in Figure 1 clearly shows a magenta colored fringe along the line. Figure 2 shows the same line after the color fringing has been removed. A halo around the power line still exists due to the high amount of sharpening used, but the magenta color is gone.
Most photographers tend to work in an RGB color space (e.g., sRGB or Adobe RGB). RGB color spaces have three channels, one for each of the three colors (i.e., red, green, and blue). Sharpening in an RGB color space can result in color fringing like that shown in Figure 1. One way of dealing with color fringing is by using LAB sharpening. LAB is simply another color space. The LAB color space also has three channels. However, the channels are different than those of an RGB color space. Rather than three color channels, the LAB color space has a lightness channel, that stores the tonal information, and two color channels, channel "a" that stores the blue/yellow information and channel "b" that stores the green/red information. In LAB sharpening, the image is converted to the LAB space and the sharpening is conducted on the lightness channel only. Since the lightness channel contains only tonal information, color fringing can not occur. After sharpening, the image is converted back to the RGB color space for any final editing and printing.
LAB sharpening is fairly easy to do. The image is first converted to the LAB color space by selecting Image/Mode/LAB Color. The Channels palette is selected. The Lightness channel is then selected (see Figure 3). The sharpening is performed on the Lightness channel using the preferred sharpening method (e.g., USM or Smart Sharpening). After the sharpening, the image is converted back to the RGB color space by selecting Image/Mode/RGB Color. Figures 4 and 5 shows a section of an image that was sharpened with this method.
The Fade Tool is another method of dealing with color fringing. It is incredibly simple. To utilize this sharpening approach, sharpening is performed by whatever method is preferred (e.g., USM or Smart Sharpening). After the sharpening has been completed, the Fade tool is immediately launched by selecting Edit/Fade. The opacity should be set to 100% and the mode to Luminosity (see Figure 6). That's it. Once the OK button is clicked, the sharpening will be applied to the tonal values only. This produces a sharpening that is almost identical to LAB sharpening, but it does not require any conversions (for those interested in the difference between lightness and luminosity, see the Lightness and Luminosity box). Thus, the use of the Fade tool does not cause the type of image quality loss that is concomitant with the use of LAB sharpening.
Still, not all is perfect. In a sense, the Fade tool is simply a bolt-on to the sharpening tool that is used (e.g., USM or Smart Sharpening). Therefore, this approach suffers from the same deficiencies as the selected sharpening tool except that the color fringing has been eliminated.
As seen in the last section, the Fade tool is pretty easy to use. Layer based sharpening isn't much more complicated. At first glance, it is so basic that it would be easy to underestimate the method. However, this would be a big mistake. Layer based sharpening is an important move in the right direction.
Layer based sharpening has three advantages. First, it eliminates color fringing. Second, the sharpening is applied to a separate layer. Like High Pass sharpening, this allows for a significant amount of flexibility. In addition, the sharpening can be undone simply by deleting the layer. While both High Pass sharpening and layer based sharpening are layer based and are sharpening methodologies, rather than tools, layer based sharpening offers much more flexibility than High Pass sharpening. With layer based sharpening, the photographer can choose which sharpening tool to integrate into the sharpening process (with High Pass sharpening, you have no choice but the use of the High Pass filter). However, by far, the greatest advantage of layer based sharpening is that it opens the door to the use of masks and filters that are the secret portal that leads to the advanced techniques and superior sharpening results.
There are actually two general ways that layer based sharpening can be carried out. In the first method, the sharpening is performed on a copy of the background layer (we will call this background-copy sharpening). This approach is good when you want to sharpen only the original image information and want the image editing to be performed on top of the sharpening. In the second method, the sharpening is performed on all of the layers (we will call this all-layer sharpening). This approach is good when you want to perform the sharpening after all of the image editing has been completed. The distinction between the background-copy sharpening and the all-layer sharpening will become important when we get to three pass sharpening (later in this series).
This procedure has created a sharpening layer upon which sharpening can now be performed (more about the actual sharpening in a bit). How much easier could it get?
It is important to understand what has been done. Essentially, we now have an exact duplicate of our original image upon which the sharpening can be carried out. A very significant point is that the blend mode has been set to Luminosity. The Luminosity blend mode only lets changes to the tonality of the Sharpening layer show through in the image. In other words, any changes to the tones in the Sharpening layer will be seen in the image, but changes to the colors will not be seen. This is important because sharpening is all about changing the local contrast (i.e., tonality) between pixels. Thus, the Luminosity blend mode will allow the sharpening that is performed on the Sharpening layer to show in the final image. On the other hand, as previously covered, sharpening can cause color fringing. This color fringing causes strange color halos to appear around the sharpened edges. Luckily, the Luminosity blend mode does not allow these color changes to show through. Thus, the effects of the sharpening will be seen, but the effects of the color fringing will not. As a result, the use of the luminosity blend mode has completely eliminated color fringing.
All-Layer Sharpening: Before the actual sharpening steps are covered, the all-layer sharpening technique needs to be introduced. The all-layer sharpening technique also starts with the Layers palette (see figure 9). In this case, there are several other layers in addition to the Background layer. For the all-layer sharpening procedure, it is necessary to create a new layer. Then, the effects of the other layers are merged into the new layer (notice that I said the layers' effects, not the actual layers themselves -- the other layers will still be there).
To merge the effects of the other layers into the new Sharpening layer, select the Sharpening layer. Then, hold down the Alt key (Option key on a Mac). While holding down the left mouse button, select Layer/Merge Visible. The Sharpening layer will now be an exact duplicate of all the layers below it. Finally, the blend mode is set to Luminosity and the Opacity to 50%. The Layers palette will now look as in Figure 11.
This layer, as with the Sharpening layer that was created in the background-copy sharpening technique, only lets tonal changes to the layer show through in the image due to the Luminosity blend mode. Consequently, the color fringing problem has also been eliminated with the all-layer sharpening technique.
Sharpening and Fine Tuning: The actual sharpening is a rather straightforward procedure. The procedure is now the same for both the background-copy sharpening and the all-layer sharpening. The Sharpening layer is selected and the layer is sharpened using one of the sharpening tools (e.g., USM or Smart Sharpening).
Now that the sharpening has been performed, it can be fine tuned. The first adjustment is the Opacity. Moving the Opacity from its original setting of around 50% to a higher setting will increase the sharpening. Moving the Opacity to a lower setting will decrease the sharpening.
Moving the shadow slider to the right determines what shadows will be sharpened. Likewise, moving the highlight slider to the left determines what highlights will be sharpened. Figure 13 shows an example. In Figure 13, the shadow slider has been moved to a value of 15. What this means is that no pixel below a tonal value of 15 (the tonal values range from 0 to 255) will be sharpened. The highlight slider has been moved to a value of 240. Thus, no value above 240 will be sharpened. Thus, both the shadows and the highlights have been protected.
However, this creates a small problem. There is now a rather abrupt transition between the areas that are sharpened and those that are not. In the shadow areas, pixels with tonal values of 15 or greater will be sharpened while pixels with lower values will not. This could be visible in the final print. A similar situation exists in the highlights. Pixels with values of 240 and below will be sharpened while those with higher values will not. Again, an abrupt transition will be created.
Fortuitously, a simple solution exists. Holding down the Alt key (Option Key for a Mac) and clicking on either the shadow or highlight slider will split the slider into two sliders (if it doesn't split at first, keep trying). The inner of the two sliders determines where the sharpening starts to be attenuated and the outer slider indicates where the sharpening stops completely. Figure 14 provides an example. The sharpening of the shadows starts to decrease at a tonal value of 20. Below a tonal value of 7, no sharpening is done at all. On the highlight side, the sharpening starts to decrease at a tonal value of 238; above a tonal value of 252, no sharpening is done.
The abrupt sharpening transition is eliminated. At the same time, a greater amount of control of the sharpening is achieved.
Layer based sharpening has some major advantages.
Nonetheless, we're still not out of the woods. The layer based sharpening that we have just covered does not address the problem that areas of different textures require different amount of sharpening. Neither does it address the fact that the sharpening requirements of the image may differ from those of the output device.
Despite these last two, as yet unresolved, problems, layer based sharpening is a significant step in the right direction. Furthermore, the full potential of layer based sharpening has yet to be introduced. The complete power of layer based sharpening comes to fruition when layer based sharpening is combined with sharpening masks and edge masks.