SO (International Standards Organization) equivalency refers to a sensor’s sensitivity to light. It is a term that is adopted from fi lm photography, when fi lm was rated depending on the way it reacted to light. A low ISO rating – or number – is less sensitive
to light, meaning it requires a longer exposure.
In contrast, a high ISO equivalency is more sensitive to light, which in practical terms means it needs less exposure. Every doubling of the ISO speed halves the brightness of light, or the length of time required, to produce the correct exposure, or vice versa. The sensitivity of an image sensor is measured in much the same way as fi lm. For example, an ISO equivalency of 200 would react to light in an almost identical way to a roll of fi lm with the same rating. Digital cameras allow photographers the luxury of altering ISO sensitivity quickly and easily. Increasing ISO sensitivity is a useful way to generate a faster shutter speed in shooting situations where you wish to capture fast action or when working in low light.