ISO help

(Taken from digicamhelp)

Auto ISO mode

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 digital cameras have an Auto High ISO mode, which is primarily used in very low and difficult lighting. The camera selects ISO number 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. Use of faster shutter speeds helps prevent blurred images due to camera shake. Using high ISO numbers also allows you to turn off the flash when shooting in dark areas.

Higher ISO means more image “noise”

Keep in mind that the higher ISO, the more noise will appear 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.