ISO Speed refers to your camera sensor’s sensitivity to light. The higher the ISO speed, the more light-sensitive it is. What this means is that you can use a quicker shutter speed, which is useful in sports photography and low light, or a smaller aperture, for where you want a wide depth of field. However, as you progressively increase ISO speed, you also increase the incidence of ‘noise’ in the image, which in turn reduces the overall image quality. In choosing your ISO speed, you therefore need to set it at a level that will ensure you get the shot – high enough to prevent camera blur, but low enough to retain maximum quality.