Task 4 How to Reduce Noise

A noisy image has tiny nondetail artifacts that are evenly distributed throughout the image. Noise typically falls into two categories: grain and digital noise. Film scanners often record the actual grains that make up the image, usually in areas of little detail and smooth tone. Digital noise is introduced either by the digital camera during the image-capture process or from post-capture processing and compressing. Photoshop CS2 is now well-equipped to handle all these problems.

1. Select the Reduce Noise Filter

With your image open, select Filter, Noise, Reduce Noise. The Reduce Noise dialog box opens.

2. Analyze the Noise

Zero all the controls by dragging their sliders to the left. Click the + button under the image preview to zoom in on an area that contains both details and even tones. Click and drag within the image to move it. You're looking for overall noise, not individual specks. Disabling the Preview check box at the upper-right of the dialog box speeds up the response.

3. Set the Strength Slider

Slowly move the Strength slider toward the right until the tiny dark and light speckles (known as luminance noise) begin to disappear. Keep the Strength setting as low as possible to protect important details. You can optionally limit the filtration to specific channels by selecting the Advanced option.

4. Set the Preserve Details Slider

Removing noise also softens image detail, but you can restore much of that detail with the Preserve Details slider. Set it as high as possible, although you'll notice a tradeoff between the amount of detail restored and the amount of noise reduced. Check several areas of the image before deciding on a setting.

5. Set the Reduce Color Noise Slider

Even after eliminating all the luminance noise, some colored speckles (known as chromatic noise) might remain. Get rid of those by dragging the Reduce Color Noise slider toward the right. Just as with the Strength slider, keep this slider set at the lowest value you can get away with.

6. Set the Sharpen Details Slider

You can sharpen the image with the Sharpen Details slider. If you're going to make further edits to the image, leave this slider set to 0 and sharpen later (refer to Part 4, Task 6, "How to Sharpen Images," for details). When you've checked several areas of the image and are satisfied with your settings, click OK.

How-To Hints

Removing JPG Artifacts

The JPG file format compresses an image's file size by throwing out image details. If an image is overcompressed, a distinctive square pattern is introduced. JPG compression artifacts are most noticeable in areas with a lot of detail. If your image has JPG artifacts, open the Reduce Noise filter, zero all the slider controls, and enable the Remove JPG Artifact check box. Click OK and do any final touch-up work with the Clone Stamp and Healing Brush tools.