Back in the days of film, for each shot on an SLR your exposure depended on setting the aperture and the shutter speed. Yeah, ISO made a difference, but you were stuck with one setting for an entire roll of film.
Fast forward to the days of digital, and you can change your ISO for every shot. Personally, this was something I had to get used to. I always shot on ISO 400 until someone said “so you’re getting noisy shots during the day when you don’t have to and having to use a tripod in low light when you don’t have to?” Oh. I see your point.
So now, if light permits I can reduce noise (or add it) by picking my ISO. I can also use ISO to change my shutter speeds without having to give up depth of field or change my aperture.
Example: I shot Pyrate Technics at Carnivale in Weed this weekend. I never changed my aperture, but I had a wide variety of shutter speeds. When I wanted to capture the full motion of the fire, I switched to 100 or 200 and shot on bulb. When I wanted a crisp photo of the dancers, I cranked my ISO to 1600 and shot at 1/60th or faster.
It’s something pretty simple, but it can really change how your pictures turn out.
First shot, ISO 1600, 1/60 at 3.5. Second shot: ISO 100, 1 sec at f5.