Digital sensors and the image processing systems have their native ISO somewhere between 80-200 ISO where the quality is at maximum. Making the sensor slower (smaller ISO) or faster (high ISO) brings a quality penalty. This is mentioned in the manuals also. The user must deliberately enable the use of slower ISO than the optimum one (Canon), or there is no possibility of slow ISOs at all (Fuji X-Pro1 has ISO 200 as at slowest). If slower ISOs are needed for long exposures, ND filters must be used.