# An overview of Noise Ninja

Noise Ninja is an effective, productive tool for removing noise and grain from digital images. It incorporates sophisticated, patent-pending technology to suppress noise while preserving detail.. The result is typically up to a two-stop improvement in effective image quality, making images more suitable for publication and enlargement.

## What is noise?

Noise is the digital equivalent of film grain. It can even look like grain, though more often it looks like ugly speckles or color artifacts. It results from a variety of sources, including sampling errors in pixels, temperature-induced "dark current" in sensor elements, and signal amplification circuits. While film grain often has a pleasant aesthetic quality to it, digital noise usually detracts from an image.

The following photographs show typical noise in digital photographs, along with the results of removing the noise using Noise Ninja:

Noise is usually most accute when a digital camera is set to a high ISO sensitivity, which is often required when shooting indoors or at fast-action sporting events. However, even at low ISO settings, noise may become intrusive when an image is enlarged. Nearly all current compact cameras show strong noise at ISO 400, and high-quality digital SLRs by ISO 1600.

Film scanners are also well known for introducing noise into digitized images, especially in dark areas of slides and in the blue channel.

Noise is an inherent property of digital imaging sensors. The laws of physics make it impossible to completely eliminate noise, and they force a tradeoff between noise levels and other properties like sensor size or sensitivity. Photons, for instance, arrive at random intervals, so the simple task of counting them during an exposure-- which is the basic function of a pixel in a sensor -- is subject to sampling error. When the exposure is shortened or the pixel size is reduced, there are fewer photons to "average out" the sampling error, so the noise increases relative to the signal.

The small sensors in compact digital cameras are more prone to noise than the large sensors used for digital SLRs. Compact digicams often have as many pixels as their DSLR brethren, but those pixels are packed into one quarter the space -- or even less. So, for any given exposure, many fewer photons reach each pixel in the smaller sensor than in the larger one, and this leads to correspondingly higher noise. So, the noise in a compact camera at ISO 200 might be the same as the noise in a DSLR at ISO 800. By the same reasoning, an 8-megapixel camera might have much higher noise levels than a 4-megapixel camera if both have the same sensor size.

## What Noise Ninja can and can't do

Noise Ninja is designed to be most effective at removing noise that is both random and uniform. "Random" essentially means that there aren't patterns in the noise. "Uniform" means that, for similar colors and tones, noise is distributed throughout the image at roughly the same amplitude. That is, the noise should not be high in one corner of the image and low in another unless the variation is due to differences in color or tone.

Noise Ninja may yield acceptable results when these conditions are relaxed. But the more the conditions are weakened, the less effective Noise Ninja is likely to be. For instance, stuck pixels, dust, and scratches are not uniformly distributed through an image -- they are localized to a few spots.

Noise Ninja can often work in conjunction with in-camera noise reduction. For instance, many digital SLRs have an option to use "dark frame subtraction" for long exposure noise reduction. This can remove a significant amount of noise that is repeatable from one frame to the next, and then Noise Ninja can be used to suppress the residual noise. Likewise, at least one popular RAW conversion tool has a simple noise-reduction feature that seems to work nicely together with Noise Ninja.