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The Big Fixx: digital-camera image problems > Dealing with Digital Noise

Dealing with Digital Noise

If you shoot in low-light situations, you’re bound to encounter digital noise. Is there anything worse than these large red, green, and blue dots that appear all over your photo? Okay, besides that “crazy music” those teenagers play, like Limp Bizkit or...well...Limp Bizkit, is there anything worse? This digital noise (often called “blue channel noise,” “high ISO noise,” “color aliasing,” or just “those annoying red, green, and blue dots”) can be reduced. Here’s how.

Step One.
Open a photo that contains visible digital noise (in this case, it’s a shot taken in low light, and those “red, green, and blue” dots appear throughout the photo).


Step Two.
Go under the Image menu, under Mode, and choose Lab Color. Switching to Lab Color is a non-destructive mode change, and won’t damage your RGB photo in any way—you can switch back and forth between RGB and Lab Color any time. You won’t see any visible difference in your image onscreen, but if you look in the title bar for your document, you see “Lab” in parentheses to let you know you’re in Lab Color mode.

Step Three.
When you’re in RGB mode, your image is made up of three channels: a Red, a Green, and a Blue channel. When these channels are combined, they create a full-color photo. When you convert to Lab Color, Photoshop composes your photo differently—although it looks the same, it’s now made up of a Lightness channel (the luminosity of the photo, where the detail is held) and two color channels, named “a” and “b.” Go to the Channels palette and you’ll see these channels. Click on the “a” channel (as shown).

Step Four.
Now that you’re affecting only the “a” channel (which consists of color data), go under the Filter menu, under Blur, and choose Gaussian Blur. When the Gaussian Blur dialog appears (shown here), increase the Radius (amount of blur) until you see the dots pretty much disappear, and then click OK. In this case, I increased the Radius to 6.2 pixels.

Step Five.
Now, in the Channels palette, click on the “b” channel (as shown). Press Command-F (PC: Control-F) to apply the Gaussian Blur filter to this “b” channel with the exact same setting we used on the “a” channel. Because you’re using the re-apply shortcut, you won’t see the Gaussian Blur dialog box—it just automatically applies the filter for you.

Step Six.
Go back under the Image menu, under Mode, and choose RGB to return to RGB mode. You’ll notice that the spots are much less pronounced because they no longer appear in red, green, and blue. You blurred the color channels, and by doing so, you eliminated those colors that are distracting to the eye. The effect appears much more muted, and in some cases (depending on the photo), will nearly disappear.

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