Notes on Noise


We’re talking about camera noise here, and specifically digital camera noise. A reader recently asked what “color noise” is and why it was objectionable in photos. “Noise” – and there are various types of noise that can afflict a digital camera – is random data you don’t want in your photos. There are different things that can generate that random data, and none of them can be eliminated completely. The way to deal with it is to try to minimize the amount of noise and shoot your photos in a way that best masks the noise that cannot be eliminated.

Color noise comparison across camera models and ISO settings, via Cambridge in Color

In this screen capture, the columns are the ISO speeds used in photographing a plain gray subject, and the rows are three different (elderly) camera models. The Canon 20D is the best of the three models of camera. Digital camera sensors have pixels that separately capture the red, green and blue light, the RGB. In this example, we are looking at the data from the blue channel, which tends to be the noisiest because it has the highest energy. You can see that, all other things being equal, higher ISO setting – the “film speed,” or more precisely the sensitivity of the camera’s sensor, the more noise. And that cheaper, older cameras tend to have more noise.

So you can see two ways to minimize noise: try to shoot at the lowest ISO possible in the circumstances and get the best camera you can afford.

What causes camera noise? In any electronic device that deals with a signal, whether the signal is light you are trying to capture, a stereo playing music, radar images or anything else, there is going to be noise. Your goal is always to minimize the noise or, to speak in electrical engineering terms, maximize the signal to noise (SNR) ratio. You can do that by minimizing the noise, by maximizing the signal or both. In digital photography, we try to do both.

In digital photography, you maximize the signal – mask the minimized noise – by getting as bright an image as possible, without blowing any part of the image. Noise is much more visible in a dark image than a light image. You can use the ISO setting to make a brighter image, but as the graphic shows, that carries a risk of more noise with higher ISO settings (although modern cameras show little noise at much higher settings than the cameras from 20 years ago). You can get a brighter image with a longer exposure interval – a slower shutter speed – although that creates a risk your subject or you camera will move and, more bad news, really long shutter speeds introduce a new kind of noise.

Here’s a photo taken in 2006 with WC’s ancient Olympus E-1 with a 300mm f2.8 lens. The shooting data: f6.3, 1/200, ISO1000. It was taken in that blue, pre-dawn light characteristic of a deep winter, high latitude morning.

Willow Ptarmigan, Creamer’s Refuge, Fairbanks

There is color noise throughout the image, even in the whites, fine to coarse speckling that badly detracts from the image quality. It wrecks what might have been a pretty interesting shot. The color noise is the result of the combination of a fairly primitive camera and (by 2006 standards) a high ISO setting. If WC had it to do over, this would have been at ISO400 and a slower shutter speed. But 16 years ago WC just wasn’t that good.

So that’s a short primer on camera noise and why it is bad. And what to do about it. For more information, check out the thorough discussion from which WC lifted that first screen shot.

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