Canon EOS Rebel T1i Instruction Manual page 61

About "AUTO" for Automatic ISO Speed
Shooting Mode
1/3/4/5/6/7/C/
d/s/f/8
2
a
With flash
* If fill flash results in overexposure, ISO 100 or a higher ISO will be set.
* If an external Speedlite is used for bounce flash, ISO 400 - 1600 will be set
automatically.
When "AUTO" is set, the ISO speed will be displayed in whole-stop
increments of 100, 200, 400, 800, or 1600. However, the actual ISO speed
may be set more precisely. Therefore, in the image's shooting information,
you may find an ISO speed like 125 or 640 displayed as the ISO speed.
If the ISO speed is set to "AUTO", the
actual ISO speed to be set will be
displayed when you press the shutter
button halfway. As indicated below, the
ISO speed will be set automatically to
suit the shooting mode.
ISO Speed Setting
Automatically set within ISO 100 - 1600
Fixed at ISO 100
Fixed at ISO 400
Fixed at ISO 400*
Z: Changing the ISO SpeedN
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