Somewhat strangely, ISO is the acronym for International Organisation for Standardization. Not that it is necessary to know this, but it might just come in useful on a quiz night! Camera film is one of the standards regulated by ISO and it specifies the speed at which film reacts to light. Without this information, it would be impossible to calculate the correct aperture and shutter speed values for an exposure.

Unlike f numbers, the ISO scale has a more logical progression. Low numbers such as ISO 100 denote a slow film and higher numbers such as 1600 faster ones. In common with f numbers, doubling or halving the number signifies that the film is twice or half as responsive respectively. Thus ISO 200 is twice as fast as ISO 100, while ISO 400 is twice as fast as ISO 200 and four times as fast ISO 100.

When digital cameras were introduced, it made sense to extend the ISO standard to sensors. It was understood and ensured consistency and continuity for those making the transition from film. Digital users benefit enormously from the facility to change instantly the ISO speeds of their cameras when faced with fluctuating light conditions. The film worker has to hold a range of stock for any situation they are likely to encounter and is faced with wasting film if circumstances are not appropriate for what is loaded in the camera at the time.

Whilst changing the ISO on a digital camera is convenient, it is normally best to leave it on one of the lower settings. Increasing the ISO value reduces the amount of light required to record an image, but to do this the signal has to be amplified by the camera’s circuitry. This means that any imperfections in the signal are magnified and manifest themselves as noise, which are random specks and colours that do not belong with the image. There are numerous applications that can remove noise, although this is always at the expense of losing some of the detail.

Digital SLRs usually perform well up to ISO 400 and the latest cameras beyond that. The technology is still developing and manufacturers are constantly introducing improvements. There are numerous camera review sites on the web and examination of these will indicate how your camera will perform at higher ISOs. Digital compacts fare less well due to their smaller sensors. That said, if the difference between getting an image or not at all, the higher ISOs are useful in an emergency.