Let's Talk ISO
Now I hear you ask a question here, if a high ISO makes a photo grainy why would you ever use that setting? Now this is a good question because in natural light (the best kind) a low ISO setting is perfect and gives the best results. However when you set your camera to a higher ISO it becomes more sensitive to low light levels. This means when it is darker in the shadows or you are taking photos at night using a higher ISO can result in more details appearing in your photo.
So remember this:
Low ISO = Less grainy photo, great for good light
High ISO = More grainy photo, great for low light
A time to use a slightly higher ISO would be if you have to shoot inside, for example at a wedding or family gathering or you are photographing your home. You may need to use an even higher ISO if you are at an evening or night party and need the maximum sensitivity to light, your photos will be a little grainier but the overall outcome will be better.
Take a look at my post on the exposure triangle if you want to see how ISO works in balance with shutter speed and aperture. Zoe x