ISO

The sensor sensitivity referred to as ASO is simply the sensor's sensitivity
                        to light.  ISO (International Standards Organization) was formally called
                        ASA in film speed. In digital cameras, one needs to set the ISO to numbers
                        from very slow to ultrafast  - like 6 to 6400 or higher.  As the ISO number doubles,
                        the sensitivity to light doubles.  An example would be an ISO of 200 would require
                        only half as much light as an ISO of 100 for the same exposure. It has been
                        reported that all cameras do not react the same to the amount of "grain" or
                        "noise" at a higher ISO. Without speaking for all cameras, it is generally
                        believed that the ISO default setting should be 200. In digital
                        cameras one can change the ISO as often as one likes.  Film cameras require
                        one to change film if one wants to change the ISO. ISO also relates to low light
                        situations and the speed of the subject such as a higher ISO needed for photographing
                        an automobile race.
                        Many of today's digital cameras have an auto control menu allowing the ISO to
                        bump up the ISO sensitivity whenever an optical exposure cannot be achieved at
                        the current ISO setting.