Removing Image Capture Noise in Channels in Photoshop

Here you will learn how to use the Noise Reduction filter on specific channels:

1. Open and make a duplicate copy of the Sadie Peak_RGB image.
2. Look through the three channels in the Channels palette to view the noise in each one. You can see that most of the noise is concentrated on the Blue channel. Therefore, this is where you should concentrate your efforts.
3. Activate the RGB composite image in the Channels palette and then choose Filter --> Noise --> Reduce Noise.
4. In the Overall tab, enter 0 for all the values, but keep the Remove JPEG Artifact option selected.
5. Now click the Advanced radio button, and click the Per Channel tab.
6. From the Channel menu, choose the Blue channel. Strength (reduce noise) and Preserve Details (sharpening) fields are presented. You know from working with the previous Palm Sunset image that you can push an individual channel pretty hard without seeing much softening of the image as a whole.
7. Apply a Strength of 10 and leave the Preserve Details amount at 0. You can see a preview of the filter’s impact on the Blue channel in the small Per Channel preview window. To see the effect on the whole image, refer to the larger preview image in the dialog box, or the image itself if you have the Preview option enabled.
8. You could also apply channel-specific noise reduction on the other channels if they required it.

Sharpening after Reducing Noise
To sharpen this image a bit, you could use the Sharpen Details tool located at the bottom of the Basic dialog box. The biggest disadvantage to using this sharpening tool with this example image is that there is no Threshold control. I recommend following up with a dedicated sharpening tool—in this example, Unsharp Mask again. Apply your Noise Reduction values and return to your RGB image view. Choose Filter --> Sharpen --> Unsharp Mask. The emphasis here is on sharpening high-contrast edges, such as the ridge and shorelines, without bringing back the noise or adding any sharpening-related mottling to the soft sky. Your approach should be to apply a large radius and a larger threshold as well. Try these Unsharp Mask values: Amount 200 Radius 1 Threshold 5 Applying these settings will make your high-contrast ridge and shorelines pop, but keep the sky soft and smooth.

Channel-Specific Softening and Printing
The first challenge when working with noisy print-bound images is making sure that you have sufficiently softened them prior to printing. Failing to do so can result in the noise pattern showing up in the print, or worse—the noise can be exacerbated by the printing process and possibly print as a moiré pattern. The second challenge is to not oversoften with noise reduction, thereby creating prints that appear too soft or out of focus. By reducing the noise in individual channels, you can remove noisy patterns while still retaining as much image sharpness as possible.

Color Shifts from Sharpening
One unwelcome result of sharpening RGB images, particularly when applied in large amounts, can be color shifts that occur along high-contrast edges. RGB images have both color and luminance values integrated into all three channels. When sharpening is applied to an RGB image, edges within an image contain different levels of contrast in each of the channels (for an example, view the differences in the contrast of the mountain ridgeline in each of the three channels of the Sadie Peak example image). This of course means that the sharpening will be applied differently on all three channels. This in turn results in the RGB values for each channel having different value shifts. In RGB mode, this results in variable shifts not only in luminance values but also in color values as well. This can create color shifts along high-contrast edges. This potential color shift can be prevented in two ways:

• Redistribute the sharpening across all three channels.
• Don’t apply sharpening to RGB images—work in Lab mode instead.

You will see how to use both of these approaches in the following examples.

1. To prevent any color shifts from occurring along high-contrast edges (the ridgeline in this example), you will redistribute the sharpening across all three channels. Choose Edit --> Fade Unsharp Mask. The Fade dialog box appears.
2. Keep the Opacity at 100%. Then click the Mode menu and choose Luminosity. This will attempt to apply the Unsharp Mask adjustment to only the luminance/lightness values in the image.
3. Save the finished image as Sadie Peak_RGB_Corrected.

Sharpening-Related Color Shifts and Printing
Color shifts can occur along high-contrast edges because of uneven application of edge contrast, which is increased when sharpening. This may become more visible when an image is printed in CMYK than when viewing the edges onscreen in RGB. Color shifts are most apparent along high-contrast edges, where there is also a large difference in color values. For this reason, it is good to get in the habit of either fading by using the Fade command (redistributing) or preventing the uneven application of sharpening altogether.