ISO, ASA… the abbreviations can make one’s head go around. What is ISO?
With film photography, ASA (American Standards Association – the older film speed rating) indicated the sensitivity of film to light. In the realm of today’s digital photography, ISO in turn refers to the light sensitivity of the image sensor. ISO (abbreviation for the International Organization of Standardization) values can range between 100 and several thousand, depending on camera models. The higher the value, the more sensitive the image sensor will be to light.
ISO speed affects the shutter speed and aperture combinations to achieve correct exposure. This means if you double your ISO from say 100 to 200 you can shoot at faster shutter speeds and/or smaller apertures (bigger f/number) because the image sensor has become twice as light sensitive. The camera only needs half the amount of light to record the same exposure.
Lower ISO settings (such as ISO100) are generally used for brightly-lit conditions, while a higher ISO (such as 1600 ISO and up) is typically used in low-light conditions. There is however a cost: the higher the ISO, the grainier your image will be. That said, the high-ISO quality of modern digital cameras is improving all the time (like with the new Canon 1dx or Nikon D4) and will no doubt continue to do so. Some modern cameras also have an ‘auto ISO’ setting and will decide on an appropriate ISO automatically to obtain an in-focus image dependent on aperture or shutter speed selected.
Back in the days of film, you may remember the grainy effect higher-ISO films produced? Grain in digital photography is better known as ‘noise’, and is a speckled disturbance affecting areas of similar colour, especially darker shadow areas. By increasing the sensitivity of your image sensor by selecting a higher ISO, this allows the sensor to record a fainter light signal. Inversely unfortunately, the increased sensitivity also allows the sensor to record more noise.
Generally, ‘noise’ is not desirable – this is why you should use the lowest ISO setting practical, and then adjust the shutter speed and aperture to get the right exposure. Image editing software such as Adobe Lightroom 4 and Adobe Photoshop CS6 have also significantly improved their noise reduction algorithms, allowing noise to be much reduced or even completely eliminated with careful post processing techniques.
The amount of light in images can be adjusted by playing with any of the following: I.S.O, shutter speed and aperture. In upcoming posts will deal in more detail with the subjects of shutter speed and aperture.
© Martin Benadie | 2012