ISO

ISO Flash

ISO 5000

ISO 400

ISO 50

ISO is the shutter speed of the camera. The higher the number the slower the shutter. Flash added its own light and made everything in a way 1 color. ISO took in the light that was already there and kept the same colors. You would change your ISO depending on how bright the object is. Also, you decide ISO if the subject is going to be moving. ISO uses the natural light opposed to flash just making everything bright. A fast ISO will be darker then a slower 1 because it doesn't allow as much light to get in.