Use blending modes to fix digital noise
Correcting images in Photoshop can be a tricky business as there's a degree of trial and error involved. Because of this, it's best to try to keep your changes to the file editable and not permanently embedded in the image. This way you can make adjustments with ease at any point in the process. Photoshop provides many ways to edit an image without permanently changing it, particularly when you're dealing with noise in an image from a digital camera. One of these techniques is blending modes, which is a layer feature of Photoshop. Using blending modes, you can set a normal layer to a variety of different blend settings to change the appearance of pixels on the layers below it. This gives you a nifty way to fix digital noise in your image without actually making permanent changes.
We'll be using the Overlay, Soft Light, Hue, and Color blending modes. We'll discuss them in groups, as some blending modes are very similar to one another.
Overlay and Soft Light
The Overlay and Soft Light modes basically do the same thing, though the Soft Light mode is more subtle. The Overlay mode either screens or multiplies colors depending on the base color below it, which means it makes your darks darker and your whites brighter. It's useful for bumping up the color contrast of an image.
The Soft Light filter also darkens and lightens the colors in the base image, but its algorithm is based on the value of the blend color rather than on the values of the base color. These details can be hard to remember, but it's easy to remember that the Soft Light mode is sort of like Overlay Light. It has the same general effect as Overlay but is less dramatic.
Hue and Color
The only difference between the Hue and Color filters is the management of saturation, or the vividness of the colors. The Hue setting applies only the hue of the blend color to the base colors. The base color retains its own saturation and luminance values. With the Color setting, the hue and the saturation of the blend color are applied to the base color. Again, you don't really need to remember the specifics. Just keep in mind that Hue typically results in a more subtle effect than Color. In general, it's best to try both when attempting to alter colors in your digital images.
Noise is a frequent problem that digital photographers have to deal with on a regular basis. Certain shooting situations tend to create images that have a lot of noise, particularly in the shadow areas or in underexposed areas, as shown in Figure A. This happens because most consumer, and even some high-end, digital cameras have a limited dynamic range that struggles in the shadow areas.
Locate the problem
Open an image that's having some trouble with colored noise. If you can't find one right off the bat, look at images you shot indoors with a flash or images that have extensive shadow areas.
Zoom in to 200% or 300% and you might see noise like our red noise in Figure B. There's actually quite a bit of noise all over the image, but it really shows up on the edges of objects where there's likely to be sharp tonal transitions. You may also notice this effect on images that have undergone a lot of sharpening or had a lot of correction.
The benefits of blur
The easiest way to correct random colored noise is to use a blurred copy of the image to knock out the noise. To try this:
Access the Layers palette and duplicate the image by dragging the Background layer onto the Create A New Layer button . The Background copy layer appears and is selected automatically.
Choose Filter » Blur » Gaussian Blur.
Set a low blur radius of about 2 pixels in the resulting dialog box. You don't typically need a large setting to fix the subtle noise we're targeting.
Click OK and set the blending mode of the Background Copy layer to Color. This applies the color of the image to your original, but doesn't affect sharpness. You can see our results in Figure C. As you can see, toning down the noise in images isn't too difficult and it results in a significantly better image.
The above method won't work for all images. On occasion, you'll find that it actually creates more general noise even as it fixes the colored noise problem. Because of this, you may need to adapt the blurring method to use locally rather than on the entire image. You can also use the Eraser tool to erase portions of the layer that are unnecessary if you think it's causing strange effects.
Change colors with ease
You can also easily remove colors with the Color and Hue blending modes. To do so:
Open an image that has a spot of color you want to remove. On the left in Figure D, you can see a brown spot at the top of the leaf that we want to remove.
Create a new layer and then sample a good replacement tone from the image with the Eyedropper tool . We selected a nearby shade of green.
Select the Brush tool and paint over the spot on the new layer.
Set the blending mode of the layer to Hue or Color to see which works best. In our case, Hue corrected the color perfectly, as you can see on the right in Figure D
As you can see, blending modes are useful for both correcting your images and adding fun visual effects. We've only touched on a few of the possible effects you can create with blending modes. With a little experimentation, you'll undoubtedly find more reasons to use blending modes when working with your digital images.