What conditions warrant the use of high ISO?
In general, high ISO is used in conditions where it is not possible to achieve a fast enough shutter speed with low ISO. Typically, the reason for desiring a faster shutter speed is to avoid blur from motion – either from the camera shake or subject motion. Situations that might require high ISO would include: Indoor, handheld shooting in available light (no flash). Shooting fast action that requires a very high shutter speed. Handheld shooting with a very large focal length. Another reason for increasing ISO is to extend flash range.
