the 100,200,400 is like in manual cams, if you have a film 100 ISO it "burns" with light so its used for outdoors and 400´s "burn" with less light so can be used indoors, but not sure if its the same in digis, also when using a 100 if you enlarge yo can see like dots, because "burning dots are bigger in 100´s" and in 400´s you can enlarge and dont see them cause they are smaller.............