Today’s Question: Is it true that sharpening can create color artifacts in photos? If so, is there a way to avoid this issue in Photoshop?
Tim’s Quick Answer: Yes, sharpening can create artifacts similar to color noise. You can compensate for this issue in Photoshop by using the Fade command in conjunction with the Luminosity blend mode.
More Detail: There are actually a couple of approaches you could use to prevent or compensate for the appearance of colored artifacts caused by applying a sharpening filter to an image. For example, some photographers convert the image to the Lab color mode and then apply sharpening to the “L” (luminance) channel.
In general I find it is simpler and easier to use the Luminosity blend mode in conjunction with the Fade command. This command can be used immediately after you have applied sharpening.
The first step, of course, is to apply sharpening to your image. I generally recommend the Smart Sharpen filter in Photoshop for this purpose, but in some cases it may be preferred to use the Unsharp Mask filter instead.
Once you’ve applied the desired sharpening to the image, go to the Edit menu and choose the Fade command. Note that this menu item will include the name of the filter you just applied, such as “Fade Smart Sharpen”.
When you select the Fade command, the Fade dialog will appear. The “normal” use of this command is to reduce the strength of the effect you most recently applied. In the case of sharpening you would generally not want to use that option, and so you would keep the Opacity setting at the maximum value of 100%.
However, in this case you do want to change the behavior of the sharpening filter you used, so you can change the blend mode to “Luminosity” using the Mode popup. This blend mode will cause the sharpening filter effect to only adjust the luminance values in the image, not the color values. The result is to mitigate any color variations that may have been introduced by sharpening.
The color variations introduced by sharpening are generally at the individual pixel level, and are not typically too extreme. However, for an image that is being printed at a large size, or for which sharpening creates visible color variations that appear as color noise especially along high contrast edges, the color artifacts can be a problem. Fortunately, the Luminosity blend mode applied through the use of the Fade command provides an excellent solution.