Noise, the digital equivalent of film grain, can be a challenge to overcome. It appears as an irregular, sand-like texture that, if small, is essentially invisible; if large, it can be unsightly and a distraction from your image. Noise can have color to it (chromatic noise) or only vary in brightness (luminance noise). (As with grain, this fine-patterned look is sometimes desirable for certain creative effects.)
Photographers generally don’t like grain in film, and the same is true for noise in digital photography. Camera manufacturers have been working hard to reduce noise at the time of capture, plus you can take some steps to reduce noise in your photographs as well.
Where Does Noise Come From?
The best way to battle noise is to understand what it is, why it happens and how to avoid or reduce it in the first place. Noise results from many causes:
Sensor Noise. Sensors always have some sort of noise, and reducing sensor noise has been one of the major research efforts of camera manufacturers. Such noise is due to several factors, including heat from the electronics and the way the sensor is put together. Sensor noise increases as the number of pixels increase within a given sensor size, although this is mitigated by the fact that noise-reduction technologies have improved as fast as megapixels have increased.
High ISO Speeds. Noise emerges when using high ISO speeds. Increasing the ISO in a digital camera is like turning up the volume control on a radio. When stations are weak, and you increase the volume, the static gets louder, too. Something similar happens when you increase your ISO speed.
Underexposure. On any camera, noise is more obvious with underexposure. Noise resides in the dark areas of a photograph. So when your entire photo is dark, and you make adjustments in Photoshop or another program, you’re allowing the noise to show up.
Heat. Sensors don’t like heat, yet they heat up with use. As sensors become hotter, noise tends to increase. This is especially true with long exposures. Increasing exposure time beyond a second or two makes your sensor work harder, increasing noise. That’s why many cameras have automatic noise-reduction features for long exposures.
Digital Artifacts. An artifact is anything that occurs in a photograph from technology and not from the scene itself. Noise is an artifact. Grain is an artifact of film. Digital imaging always has had trouble dealing with fine gradations, such as those found in the sky. That trouble with infinite gradations is because a pixel is either on or off. This sometimes shows up as noise in certain areas.
JPEG Artifacts. These artifacts are caused by image compression and reconstruction of an image when it’s opened in the computer. Higher levels of compression result in more JPEG artifacts.
Avoiding Noisy Exposures
To counter noise and its effects, you need to start when you first take the picture. Many of the causes of noise can be controlled as you photograph. You can’t change your sensor, nor can you change the way the sensor deals with long exposures, but you can control many of the other factors. Here are some ideas:
1. Avoid underexposure. With today’s cameras, sensors are doing a great job of controlling noise and keeping it to a minimum. But as soon as exposure goes down, you can run into trouble with noise. Sometimes people say they don’t worry about exposure because they shoot RAW. RAW files won’t necessarily reduce noise. Exposure is important no matter what format you’re using.
Just because you don’t have blinking highlights on your LCD doesn’t mean you have a good exposure. That simply can mean that you’re well underexposed. What you can do is watch your exposure and look for blinking highlights—the warnings of overexposure. Use an exposure that just barely makes the blinking highlights go away.
A better sign to look for would be no gap at the right side of your histogram. If you see a big gap between most of the histogram and the far right side, this is often an indication of underexposure.
2. Use low ISOs whenever you can. Be careful not to go overboard with this. I’ve had students turn in pictures that were blurry because they used too slow a shutter speed as a consequence of too low an ISO. It’s better to have sharp pictures that have some noise than to have blurry pictures with no noise. Still, when you can, use lower ISO settings, especially if you can use a tripod or other support.
3. Turn on noise reduction for long exposures. When you turn on noise reduction for long exposures, you’ll find that your exposures take more processing time before you see the results in your LCD. Typically, this time will be double your actual shutter-speed time; if you had a one-minute exposure, you might wait two minutes before you see an image. Sometimes photographers don’t like this, so they turn off noise reduction. However, noise reduction for long exposures works and reduces the time spent later removing the noise.
4. Keep your camera cool. At times, we photograph in hot conditions. Look for opportunities to keep your camera from getting too hot; that means not leaving a black camera in the sun. If your camera is on a tripod or sitting out on the table, put a hat over it.
5. If you shoot JPEG, use the highest-quality setting. This minimizes any problems you might have with JPEG artifacts.
Working With Noise In The Computer
No matter what you do, you may end up with noise in your pictures. But you can reduce noise in the computer and increase its appearance as well; so we need to address that issue.
1. Sharpening. When you use Unsharp Mask and find noise to be an issue, set the Threshold to a number between 6 and 12. Smart Sharpen in Photoshop is problematic because it doesn’t have a Threshold setting, so it’s not the tool to use when you have noise issues.
Another technique is to selectively sharpen your photograph. Copy your photo to a new layer, sharpen that layer and remove the parts that are causing noise problems. (See the article “Selective Sharpening” at www.pcphotomag.com for more details.) Photoshop Lightroom also includes a masking slider for its sharpening tool that can reduce the sharpening of noise (hold down the Option/Alt key to see where it affects the photo; white is the sharpened area).
2. Color and exposure adjustments. Over-enhancing the color of skies can increase the effect of digital artifacts, which makes the sky have more noise. You need to be careful when using the Hue/Saturation adjustment, because this often can increase noise quickly if it’s set too high.
e you have noise in a photograph, you can reduce it with special programs. Adobe products have some noise-reduction tools built in. You can find them in Photoshop under Filter > Noise > Reduce Noise; in Lightroom, go to the Develop module under Detail. Many other programs also include noise-reduction tools. These generally work fine for minor amounts of noise, but if you have very strong noise, you’ll probably prefer the results of specialty software designed to deal with it.
By using noise-reduction software, I’ve been able to make a photo with a higher ISO setting look like it was shot with a considerably lower setting. I’ve also been able to use cameras with smaller sensors and have the results equal the results from cameras with larger sensors.
I haven’t used all the noise-reduction programs on the market, but I’ve found that Nik Software Dfine 2 works very well. Version 2 is a complete redesign, and it’s extremely easy to use. I’ve been impressed with how well it removes noise from a picture without hurting the sharpness details of the photo. Noise is essentially small details, so if you reduce it, you often reduce other small—but important—details for sharpness. In addition, Dfine has advanced features that allow you to selectively control noise in certain areas. For example, you can tell it to reduce the noise in a blue sky without affecting detail in other parts of the photograph.
Selectively Reducing Noise
You may find that noise appears only in certain areas of an image. This is common with a well-exposed picture that has dark areas that need to be brightened. When the dark areas are brightened, noise shows up.
The solution is to selectively control the noise. Copy your image to a new layer, apply noise reduction to the whole thing and then remove everything from that layer except the problem area with the noise. You can do that by erasing the parts of the picture that don’t have a noise problem or by using a layer mask.
Follow the tips in this article, and you’ll begin to control noise in your photographs. As you work with noise, you also may find that some images look better if you actually add a little noise. This is a whole different story, but it can be a fun way to work with photos, especially if there’s a lot of noise in the images to begin with.
Clean up noisy images with Nik Software’s Dfine 2.0, a Photoshop plug-in that lets you selectively eliminate noise. Using U Point technology Control Points, you can constrain adjustments to just where they’re needed. List Price: $99. Contact: Nik Software, www.niksoftware.com.