iso is not abt allowing more light in but just the sensitivity. depends on how dark, i dont like to raise the iso too high in lit places as there would be more noise..
the lower the ISO, you either have to have a longer exposure (i.e. slower shutter) or lower F-stop to let more light in.
let me give you two examples..
this picture was done at F10, 3 seconds, iso 400 (notice the street light at right corner)
for an equivalently bright picture...
this picture is done at F3.5, 1/3 seconds, iso 400 (notice the diff in the street light right corner)
so its how you toggle between the F-values and the shutterspeed..
a higher F-value i.e. above 10+, those streetlamps and other bright spots will be more radiant, rather than a big blotch of overexposure..
the only time i used iso 1600 or higher was for astrophotography.. when the place is like totally dark with minimal lighting, i.e. even the dark night sky was like daylight..
higher isos are used indoors, for portrait shots etc as well because must pictures of events and pictures, you cannot use too slow a shutter speed. so higher sensitivity, allow for you to use a higher shutter speed without the picture being underexposed. the same for F-stops.. those super low aperture lenses, i.e. F1.8, F1.4 allow more light in, and thus you can use a faster shutter as well, at times w/o increasing the iso too much..