|Image with 2 different ISO setting|
High ISO values allow you to shoot with fast shutter speed to achieve optimal exposure with given aperture. High ISO also allows you to shoot under low light, but at the same time it also produce grainy image and reduces the quality of your image. Low ISO setting produces high quality images but limit the use of higher shutter speed. Choosing ISO value is all depends on under which circumstances you are photographing and how much you can compromise with the quality of your image.
In the above Image, you can see clear grains photograph at high ISO setting i,e. ISO 6400 while the another image ISO 100 have no grains and have better quality. The shutter speed for the Image ISO 100 and ISO 6400 was 1/160 and 1/25 respectively.