Auto ISO modes
All consumer digital cameras have an Auto ISO mode. The camera automatically selects the sensitivity, known as ISO, according to the level of light in a scene. The user has no control over which ISO number is used.
In bright light a lower ISO will be selected. In low light an higher ISO number will be selected.
Auto high ISO mode
Some compact digital cameras have an auto high ISO mode that is primarily used in very low and difficult lighting. The camera selects ISO numbers higher than auto ISO mode, usually ISO 400 and above.
Higher ISO means faster shutter speeds
When a higher ISO is selected, shutter speeds are faster and apertures open wider, allowing more light to enter the camera. Use of faster shutter speeds helps prevent blurred images due to camera shake. When using high ISO numbers, you may not have to use a flash.
Higher ISO means more image “noise”
Keep in mind that the higher ISO, the more noise appears in images. That’s why many photographers, in order to prevent noise, prefer to set ISO manually and select low ISO numbers even if they must use a tripod to prevent camera shake. Still, an photo with noise is better than no photo at all.
Image noise will be higher when using Auto High ISO mode than using Auto ISO mode. Whenever possible, manually select ISO and use the lowest ISO setting possible.