You might have noticed that under certain conditions, the image acquired with your DSLR has far too much information, thus creating noise and digital artifacts that mar your photo. This is particularly an issue when you're shooting at night and/or with a high ISO in other low-light conditions... it's the downside of attempting to capture an image when the exposure latitude (the difference between the lightest and darkest section) is wide. So Photoshop has the ability, when properly applied, to eliminate a tremendous amount of that noise. Here are a few methods to do it.
The Noise Reduction Filter is an excellent means of eliminating unwanted noise in photos that were taken at a high ISO, because you have control over the luminance and color noise, also you can address the noise issues on a per-channel basis. Here’s how it’s done. To begin, open up your photo and then double-click on the Zoom Tool to bring your image to 100% (always work at least 100% when editing so you can truly see how you’re manipulating the image).
In the Layers palette, Right Click on the Layer and Convert to Smart Object. By converting to a Smart Object, you will maintain all of the images original quality and its resolution, so if you change the image (by scaling, moving, skewing or warping it… all under the EDIT > TRANSFORM) Photoshop doesn’t discard any of the pixel information that is sometime lost through these manipulations. In essence, the final rendering is completed when you finally commit to it (flatten or output the image), not when you make the initial change. This gives your much more control – for example if you change your mind several steps down the editing road.
Next, move on to FILTER > NOISE > REDUCE NOISE. In the new window, navigate around the image in the Preview window to find the area that you need to eliminate the noise artifacts from, and increase the Strength. This allows you to remove luminance noise. You’ll also want to increase the Preserve Details, because you don’t want to loose any details. However, you might have to if the image is really “noisy”. Next, tweak the Reduce Color Noise and Sharpen Details sliders to get the best results. Reduce Color Noise eliminates that confounding color mottling that you see, but keep the setting under 50% otherwise you’ll desaturate your image too much. If you really must go higher than 50%, add a Hue/Saturation adjustment layer to bring the saturation back. Sharpen Details… be very wary of this slider, as you might get unsightly edge artifacts when you crank it up. If you’re working with a low resolution JPEG file, then you’re going to have JPEG-inherent artifacts (blocky sections and/or halos), luckily most of these can be eliminated with Remove JPEG Artifacts button.
If you really have stubborn noise, you’ll want to access the individual channels to knock out the specific noise. Click on the Advance button so you can alternate between the Red, Green and Blue channels. This is important because the Blue channel usually contains the most luminance noise. In a night shot with considerable ISO or exposure noise, you’ll want to attend to the Red and Green channels, because they stand out the most from the dark night sky and your shadows. What you have to remember is that alterations in the Advance mode are applied on top of the other changes you’ve made, so plan to respond accordingly.
Before & After Noise Reduction
In general, you’ll want to eliminate the noise in all your photos, and because of the nature of digital photography, almost every image has some noise that needs to be eliminated. The Noise Reduction filter is ideal for doing that, so it’s best to get familiar with it.