newbie at iso

I've had my camera for about 4 months now figured out all the options
f/stops and shutter speed only thing that puzzles me is iso how do you know what iso to shoot at i usually keep at 100 to prevent noise but is there an advantage to shoot at a higher iso?

thanks

The general practice is to keep it at 100, and raise it if you can't get a fast enough shutter speed for your given aperture. Let's say you're shooting a person running, and it's late in the day, and even with your Nifty at f/1.8, you can only get say 1/30sec shutter speed. If you want to stop the motion of the runner, you'd raise the ISO to around 400, which would give you 1/125sec.

It sounds as though you understand exposure as a two-legged relationship. Faster shutter speed = lower aperture number and vice-versa. But the reality is that exposure is three legged, ISO, aperture and shutter speed.

So if you fix one (suppose you need a shutter speed of 1/400 for sports) then you can adjust the other two to get the right exposure. Every full stop smaller aperture (f/2 to f/2.8 for instance) will require one full stop higher ISO for the same shutter speed (like ISO 200 to ISO 400 for example).

People usually prefer high shutter speed to freeze both camera shake and subject motion - though sometimes you actually want to include blur to show movement like in star trails and waterfalls.

People usually prefer lower ISO to avoid noise

Aperture varies depending on the depth of field desired.

Every shot demands balancing the above concerns and compromising when needed.