To be more technical, as ISO goes up it is the equivalent of shortening your shutter speed. When you go from ISO 100 to ISO 200 it has the equivalent of shortening your shutter speed by one stop or, in terms of aperture, it opens up the aperture by one full f-stop (it doesn’t change your depth of field but has the effect of letting more light onto the film).

The scale goes like this:

ISO Number Number of Stops Changed
ISO 50 = +1
ISO 100 = 0
ISO 200 = -1
ISO 400 = -2
ISO 800 = -3
ISO 1600 = -4
ISO 3200 = -5
ISO 6400 = -6

 

Most point and shoot (non-DSLR cameras) go from ISO 100 to ISO 400. Decent DSLRs like my Nikon D80 tend to go from ISO 100 to ISO 3200. The incredible pro DSLRs go all the way to ISO 6400.

When in doubt turn the ISO up if you think that’s the only way you’ll get the picture you want. But, in the long run you will want to do you best to turn the ISO rating down so that you get good clear pictures. If you don’t have a tripod and want a night picture then turn up the ISO but you will be much happier with a picture taking t ISO 100 on a tripod.