Canon EOS Rebel XTI - Manual (Page 53)

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Set the ISO speed (imaging sensor’s sensitivity to light) to suit the
ambient light level. For example, when you increase the ISO speed
(higher number) for low light, a faster shutter speed can be used and
camera shake will be less prone to occur. The effective range of the
flash will also increase.
1
Press the <WZ> button.
X [ISO speed] will appear.
2
Select the ISO speed.
Press the <S> key to select the
desired ISO speed, then press the
shutter button halfway.
ISO Speed Guide
Z : Changing the ISO SpeedN
ISO Speed Normal (No flash) Flash Range
100/200 Sunny outdoors
See page 54 400/800 Overcast skies, evening
1600 Night or dark indoors
Using a high ISO speed or shooting in high-temperature conditions may
result in slightly grainy images.
High temperatures, high ISO speeds, or long exposures may cause
irregular colors in the image.
In the Basic Zone modes, the ISO speed will be set automatically within
ISO 100 - 400 to match the respective light level.
With ISO 800 or 1600, camera shake is less prone to occur than with the
Basic Zone modes. The flash range will also be longer.