Photography Tech Tip : ISO and noise
This is one in a series of 'Tech Tip' articles intended as a guide to digital camera technology and terminology. Its aim is to demystify some of the language used on this site to allow you to make more informed decisions when considering purchasing a new digital camera.
ISO
In relation to digital photography, the ISO (International Organisation for Standards) number, refers to the sensitivity of the digital camera sensor to light, and is derived from the same term relating to film photography. It is therefore often referred to as equivalent ISO in digital photography in reference to the original film standard.
Historically, in film photography, the ISO number was sometimes referred to as the "speed" of the film as it dictated the amount of time required for correct exposure:
- higher ISO values = shorter exposure times = 'quicker' film
- lower values = longer exposure times = 'slower' film
The principle translates directly to digital photography. Higher ISO values increase the sensitivity of the sensor, requiring less exposure (see Tech Tip: Exposure) to create an image. The ISO number is inversely proportional to the exposure and is on a linear scale, which means that the same exposure can be achieved in half the time at a setting of ISO 200 than of ISO 100.
ISO levels on modern digital cameras range typically from ISO 100 to 800, with ever broader ranges available with the advance of technology. A typical dSLR camera will offer a range from 100 to 1600, for example, and very high ISO levels are popular as a means of image stabilisation in compact cameras because of the increased shutter speed achievable through their use.
NOISE
However, the use of high ISO levels has a cost. As the ISO level is increased, so is the occurence of 'noise' on the image. The 2 main types of noise are:
- random
- fixed-pattern
Random noise is exactly that - random - and appears as a grainy texture on the final image very similar, in fact, to the same effect seen in high ISO film. In digital cameras, this can be caused by electrical fluctuations on the sensor during exposure or during image processing. The pattern of the noise will differ for every shot, making it difficult to remove automatically.
Fixed-pattern noise is caused by damaged photosites (see Tech Tip: Sensors) on the sensor. These will record only one colour, or are extra sensitive, which causes bright or mis-coloured dots to appear on the image. The difference between this and random noise is that fixed-pattern will be consistent across all images. This makes it relatively easy to remove and many cameras use a noise reduction system that subtracts the fixed-pattern noise (determined through a closed-shutter exposure to identify bright dots against a black background) from the final image.
NOTE: Using noise reduction for long exposures can have some side effects, as discussed in our news article 'Extended processing times for long exposures'.
The pics below show 4 different shots taken with different settings so you can see the difference (thank you, Mr Wasp, for sitting so patiently while I fiddled with my camera!):
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1/100sec shutter speed; ISO 100:
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1/400sec shutter; ISO 400
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1/800sec shutter; ISO 800
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1/1600sec shutter; ISO 1600
All images have been blown up from the original, so they're actual size, cropped from the original 3.8k x 2.5k pixel images. You can see the degradation in the image quality as the ISO level is increased to 1600 (compare the dark areas around the eyes and on the back), but the exposure of the picture remains constant by increasing the shutter speed in proportion to the increas in ISO.
