Canon DIGITAL IXUS I5 User Giude page 151

ISO Speed (p.87)
The ISO speed is the numeric representation of the camera's sensitivity to
light. The higher the ISO speed, the higher the sensitivity. A high ISO speed
allows you to shoot images in dark indoor or outdoor conditions without a
flash and also helps prevent blurred images due to camera shake. This is
convenient when shooting images in places where flash photography is
prohibited. The ISO speed setting makes full use of the available light,
resulting in images that convey the feel of the place they were shot.
ISO 50 equivalent ISO 400 equivalent
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