Reducing Color and Luminosity Noise

Noise can be quantified as unwanted luminosity or color data, also known as chrominance signal-to-noise ratio. Noise is mostly apparent in images taken with consumer cameras, but prosumer cameras are also prone to it. It can appear as random patterns of color, most noticeable in light areas, such as skies, and dark areas, such as shadows.

Noise is a byproduct of a number of factors. It can be attributed to heat generated by sensors, the proximity of photosites (small sensors crammed with too many photosites), images taken with a high ISO setting, or long exposures. Typically, the green channel tends to exhibit less noise than the red or the blue, the latter being the noisiest of the three.

Photoshop has a new filter for counteracting noise: Reduce Noise. It does a good job of removing noise and JPEG artifacts while preserving edge detail. It has two modes: Basic and Advanced. In Basic mode, noise reduction is applied to all channels. When it's set to Advanced mode, you can apply per channel noise reduction.

To use Reduce Noise, take the following steps:

1. To see which channel has the most noise, click the separate channels one at a time in the Channels palette. Most likely, it will be the blue channel. Select the composite channel to return to composite view.

2. Choose Filter ^ Noise ^ Reduce Noise.

3. Accept the default settings or move the sliders until you strike a balance between noise reduction and detail loss (Figure 11-19). The Preserve Details slider removes the most luminance noise when set to 0% and the least when set to 100%. The Reduce Color Noise slider removes random color noise as expected: Higher percentage values remove more noise. The dialog box has a good-sized proxy window, which you can click to see a before and after view. As with most filters, you can also toggle the view in the document window by clicking the Preview checkbox.

4. Click the Advanced radio button and then review the noise in the separate channels by selecting a channel from the Channel pop-up menu.

5. Move the Strength slider to specify a noise reduction in the chosen channel (click in the B&W proxy window to see a before and after view).

6. If you think you will return to the same settings in future sessions, you can save the settings by clicking the button next to the Settings pop-up menu. Give the settings a memorable name in the pop-up dialog box and click the OK button to return to the Remove Noise dialog box.

7. Click the OK button to exit.

Figure 11-19: The Reduce Noise filter.

Bearing the following pointers in mind when using Reduce Noise can help:

■ If you're processing files in Camera Raw, turn off Color Noise Reduction and Sharpness under the Detail tab.

■ Try not to overdo the noise reduction. In some circumstances, high settings used in Camera Raw could result in blotchy channels that might not separate as expected.

■ Apply any adjustments that might increase noise, such as Curves moves or Shadow/Highlight, before applying Reduce Noise.

■ Inspect the channels before you enter Reduce Noise. Use the channel controls available in Advanced mode if your image has greater noise in one channel (usually the blue channel).

■ Apply minimum sharpening in Reduce Noise; use Smart Sharpen or Unsharp Mask to apply sharpening.

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