Got Noise?

Even using a high end SLR with a large sensor digital noise can occur at high ISO settings causing pinkish, pixelated speckles which can detract from an image. 

There are a number of sources of noise:

• Heat generated might free electrons from the image sensor itself, thus contaminating the "true" photoelectrons. These "thermal electrons" give rise to a form of noise called thermal noise or dark current.
• Another type of noise is more akin to the 'grain' obtained by using a high ISO film. When we use a higher ISO, we are amplifying the signal we receive from the light photons. Unfortunately, as we amplify the signal, we also amplify the background electrical noise that is present in any electrical system.
• In low light, there is not enough light for a proper exposure and the longer we allow the image sensor to collect the weak signal, the more background electrical noise it also collects. In this case the background electrical noise may be higher than the signal.

So why is using a larger image sensor better? Each photosite on the sensor itself generates electrical noise that can contaminate its neighbor. In a larger image sensor, the photosites can be physically further apart and thus be less affected by that contamination. A larger image sensor also means that the photosite can be larger, thus have a larger light gathering capacity. It is therefore able to generate a larger signal to noise ratio. That is why a digital camera with 6 million pixels crammed into a 1/1.8 in. image sensor has more noise (especially at high ISOs) than a 6MP digital camera using the much larger half-frame (APS-sized) image sensor.

Let's look at an example. The photo below was taken in a very dark internal room of the Preah Khan temple in Siem Reap, Cambodia where I came across a lovely old lady praying. Using only available natural light I shot this image at ISO 3200.

Although the image is still good, especially given the poor lighting conditions, there is visible noise in the image which is more obvious when zoomed in as shown in the image below. You can see the pixelation particularly around the background pink coloured jos sticks just to the left of the lady's face as well as in certain areas of the lady's face.

Noise Ninja is a very effective and productive solution for removing noise and grain from digital photographs and scanned film images. It is a must-have tool for anyone shooting in low-light or fast-action situations -- including news, sports, wedding, and event coverage -- where high ISO photography is required and the resulting noise compromises the image.

Noise Ninja often yields a two-stop improvement in effective image quality, while preserving important image detail. In addition, it can produce cleaner, smoother enlargements from low-ISO images.

Noise Ninja can be used as a plug-in to Aperture or as a standalone application. Using simple slider controls as shown above you can adjust the luminescence strength and the colour (chroma) strength plus carry out sharpening whilst visualising the results of these adjustments on-screen. The picture below shows the original photo above and the noise reduced image below.

The final noise reduced full size image can be seen below.