The general idea is if your scene is well lit, such as an outdoor shot on a bright day, you should keep your ISO low, around 1-200. If your shooting in the dark, be it outdoor or a dimly lit room, you will need to up your ISO.



However, like everything else there are the pros and cons to this feature. Essentially, while you will get a well illuminated image when needed, the higher your ISO setting the more noise you will get in the image.

This noise can be more noticeable depending on the conditions but can sometimes be removed in post process.

So remember, you'll always have that trade off between noise and ISO but you never know, noise may add to the scene, it all depends on what your looking for.