Canon PowerShot 5D550 Digital Elph User Manual page 94

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Raise the ISO speed when you wish to reduce the effects of camera
shake, turn the flash off when shooting in a dark area, or use a fast
shutter speed.
Press the 150 button and switch settings.
eCycle through the ~/~/~/~/Auto ISO speed with
each press of the
ISO
button.
eSelecting [Auto] sets the sensitivity to produce the optimal
image quality.