What is… Image Noise? [video]

Matt explains what image noise is and how you can tackle it



You’ve probably already seen image noise as the coloured speckles you see in shadows, the grain you see over images taken at night. But why does it affect images and what can you do about it?

Noise is any kind of deviation from what’s expected at the pixel level. So, instead of recording a mid-grey, for example, your camera may record something slightly darker or lighter shade. If this happens on a significant enough scale, you end up with a visible texture which we refer to as noise.

It’s usually easier to spot in flat, featureless areas of an image such as skies rather than in scenes with more intricate details. The reason for this is that busier scenes effectively hide some of the noise that forms, whereas with areas of little detail fail to do the same.

There are a number of reasons why noise forms, some of which you can’t do anything about. Some noise, for example, forms because of how light arrives at the sensor and some is generated simply from converting the charge at each pixel into digital information.

Sensors also produce more noise in warmer conditions and as they heat up through use, so you should bear this in mind when shooting in particularly hot conditions.

There a number of things you can do to minimise noise appearing in your images. As a general rule, using the lower ISO settings available will lead to less noise forming, so only use the higher ones when you have to. If your camera has an Auto ISO setting, it may allow you to set a maximum sensitivity so that you can avoid using sensitivities past a certain point.

Instead of using a high sensitivity, you could try using a wider aperture. This will let more light pass through to the sensor and will stand to create a cleaner image than a high ISO setting, although you should make sure a wider aperture results maintains the depth of field you want in your image.

You could also try a long exposure instead of a high ISO setting, although you’ll need to make sure your camera is absolutely stable on a tripod or a similar support. It will also blur any movement in the scene so make sure this is the effect you want.

Another thing you could try is flash, either from small pop-up models built-in to most cameras or through a separate unit. This won’t be appropriate for every image although with more light to work with your camera won’t need to work as hard to record fine details in shadow areas, and so it’s likely to help.

Most cameras also come with option for noise reduction, and this is usually effective for images captured at high sensitivities and long exposures. It’s a good idea to keep long exposure noise reduction on as it’s generally effective without degrading images, although you might want to limit high sensitivity noise reduction as the stronger settings can rob images of their detail.


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