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July 14, 2007
[ KEEP THE NOISE DOWN, CHAPS! ]
DPEXPERT HAS BEEN RABBITING ON LATELY about image noise in the cameras we have been testing. And readers have been writing to us, perplexed by the concept of visible noise. Isn’t noise something that you hear, and not something that you see?
Image noise looks like film grain. It shows up as a speckling or mottling of flat surfaces, such as the sky. It can be a particular problem in slightly underexposed areas, as in the shadows on the side of a nose. When it is really evident it can obscure fine detail, such as individual hairs.
A number of factors affect the noisiness of a picture. Long exposures are more prone to noise than short ones. The higher the ISO setting on the camera the more noise there will be. The higher the density of photoreceptors on the sensor chip the more noise is likely to be intrusive, even at relatively low ISO speeds.
As manufacturers fit their compact cameras with tiny sensors sporting 10 million pixels noise is becoming more of an issue. Why should this be so?
A camera sensor is made up of millions of light sensitive diodes which convert the light into an electrical signal that will be interpreted by the camera’s processor as a colour – red, blue or green. The receptors generate some signal even when there is no light falling on them and that is noise. And, unlike film grain which is black, digital noise is coloured. Camera makers must make sure that their pixels generate enough charge to render the underlying noise invisible.
The bigger the pixel the easier it is to get a good signal to noise ratio. So professional cameras, like the Canon 1D Mark III reviewed here, have 10 million big pixels on an area of 28 by 19 mm. The Canon G7, the company’s top compact, also has 10 million pixels, but on an area 7.2 by 5.3 mm. Not only are the pixels smaller, the space between them is also decreased and minimising noise is more difficult.
Some cameras have inbuilt noise reduction but this also creates problems. If the effect is too aggressive skin will have all its detail smoothed out and will look like plastic.
Most cameras produce acceptable prints up to the standard 10 by 15 cm size, and image noise is not an issue. But for photographers with A4 or A3 ambitions noise may be a problem, particularly if the final print is a cropped part of the original.
Not only is more not better where noise is concerned, ten megapixels in a compact camera may not even give better image detail than a six megapixel sensor because of the limits on the resolving power of the lens.
There are noise reduction programs that do an excellent job of removing the grain from pictures, but most users of compact cameras are not going to edit images in the computer. It is better to resist the sales person who promises more pixels for fewer dollars and to insist on seeing the best six or seven megapixel camera on offer. While we’re talking about Canon product we love the Ixus 800is ($600). It has a six megapixel sensor and an image stabilised lens, plus an optical viewfinder. Very sensible.
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Posted by terry at July 14, 2007 02:36 AM
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