[COLOR=darkgreen][B]No.6[/B][/COLOR][COLOR=black]: Image noise - what is it and what causes it?[/COLOR]

Noise is the scourge of the modern day photographer. It used to be called '[B][COLOR=darkgreen]grain[/COLOR][/B]'. Noise is an inconsistency in the values of pixels in areas of the image that should be exhibiting smooth graduations in tone. Noise is usually associated with images taken in low light and at [COLOR=darkgreen][B]high ISO[/B][/COLOR] sensitivity settings. But noise can also bee seen in pictures at low ISOs where post processing has boosted the levels in areas of the the picture. By making large changes to the tonality, you increase the error values inherent in pixels and these become visible as noise.

Cameras with smaller sensors or, more importantly, sensor [B][COLOR=darkgreen]photosites[/COLOR][/B] (the millions of pits that are photo-sensitive) the higher the likelihood is that an image produced will be noisy, which is why large sensor DSLRs are favoured by photographers requiring noise-free images.

Blue areas of an image are usually more prone to noise than other colours, so check out those blue skies in your photos!

There are two main forms of noise; [COLOR=darkgreen][B]luminance noise[/B][/COLOR], and [COLOR=darkgreen][B]chroma noise[/B][/COLOR]. Luminance noise, which is more closely associated with old-fashioned 'grain', has less of a colour component to it than chroma noise.

[URL="http://dpnow.com/galleries/showphoto.php/photo/18520"][IMG]http://dpnow.com/galleries/data/766/ISO100.jpg[/IMG][/URL]

The above section of an image was shot at ISO 100. There is no noise to be concerned with.

[URL="http://dpnow.com/galleries/showphoto.php/photo/18521"][IMG]http://dpnow.com/galleries/data/766/ISO3200.jpg[/IMG][/URL]

Here is the same are of the scene, this time shot at ISO 3200. There is much blotchy artificial colouring that is characteristic of chroma noise. Banding in dark areas of a high noise image may be present, which is sometimes referred to as structured noise.

[COLOR=black][URL="http://dpnow.com/galleries/showphoto.php/photo/18522"][IMG]http://dpnow.com/galleries/data/766/Luminance-noise.jpg[/IMG][/URL][/COLOR]

Above, the chroma noise has been filtered out and we are left with a textured graininess that is not too objectional; this is luminance noise. It's also easier to dial out luminance noise with utilities designed to deal with noise.

To avoid noise, the best advice is to get the exposure spot on. This means there is no need to manipulate the tonality of the image later and cause secondary noise.

Keep your ISO setting as low as possible. Shooting in RAW mode, if your camera supports this, can give you more headroom for dealing with noise. If noise is present, sharpening an image will amplify it.

Sometimes noise looks more objectionable on-screen than when printed and this is especially so of chroma noise. Indeed, some photographers actually add luminance noise in post-processing to give some images extra depth for printing. Remember, the normal viewing distance of a print will often mean that your eye no longer resolves the noise, leaving you a smooth view of the image.

Recommended third party utilities for filtering noise include Noise Ninja, Noiseware, and Neat Image.

[B][COLOR=darkgreen]Tomorrow:[/COLOR][/B] Lens sweet spots.

[I]Incidentally, please don't hesitate to post a question about this Daily photo tip if you have one![/I]