The digital cameras allow an extension of their native ISO range either by giving opportunity to increase amplification of the signal (commonly referred to as 'Hi' setting, for example, on the D5 Hi-1 is 204.800 ISO equivalent), or by attenuation the signal (the 'L' or 'Low' setting). In the latter case, noise is slightly reduced because the output is compressed, in the first case, noise increases because of the amplification. In either case you lose dynamic range.