A Viewfinder Darkly
Phil Northeast is Feature Writer for Photography at 
Stock photography by Phil Northeast is available from Ozimages International
ISO Speed v Digital Image Noise
Philip Northeast
Digital cameras share the issue of increasing light sensitivity leading to decreasing image quality with film photography.
Before noise reduction at ISO 3200
To make film more sensitive to light one of the things manufacturers do is to increase the size of each tiny film sensitive area or grain. More sensitive, or faster film, tend have a more noticeable graininess when enlarged. Film is manufactured with different light sensitivity ratings conforming to the international standard set by the International Standards Organization, hence the acronym ISO.
The ISO light sensitivity rating carries over to digital cameras. Here, as in film, the higher the ISO number the greater the light sensitivity setting. One of the advantages of Digital cameras is the ability to vary the ISO setting for each shot. While in their film cousins, a roll of film has a fixed ISO rating. In digital cameras, the ability to alter ISO rating is a means of exposure control, along with aperture and shutter speed
After noise reduction at ISO 3200
The down side to high ISO rating, or more sensitivity to light, is an increase in electrical noise. The effects on the image are comparable to the grain effects in film images. Digital Noise shows up in darker areas as a granular texture with random speckles.
When light strikes a digital camera’s sensor, it gives electrons energy and they flow in the sensor, the more light the more electrons that flow and this is the basis for digital imaging. However, a small number of electrons will flow naturally and randomly. In extreme low light situations the number of naturally flowing electrons are not much different to the number of electrons flowing due to the light and becomes more noticeable.
Noise Reduction Software:
As in film images, the larger the digital print the greater the effect of noise. There is help at hand, as there are a number of software solutions to reduce the noise in your digital images, here are two examples.
Noise Ninja from picturecode.com offers a range of options from plug-ins to stand alone versions of their noise reduction software. There is a professional standard version that includes a batch processing option for when you have hundreds of images from a days shooting to process, very handy. They offer a trial version that allows you to produce before and after versions of your images, although it does produce a watermarkimage.
There is an alternative, a free program ndnoise that requires a java runtime environment. If you only need to do a few images then it is worth a try.
When images are resized for web or email use, the noise becomes less noticeable, so if this the main use then there is no need to special software. However, to produce the best quality large size prints then some form of noise reduction will help.
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