Pentax K-5 Operating Manual page 102

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You can set the sensitivity to suit the brightness of the surroundings.
The sensitivity can be set to [AUTO] or within a sensitivity range equivalent
to ISO 100 to 12800. The default setting is [AUTO].
I
Turn the rear e-dial (;,,_,,_) while
pressing the ISO button in
Capture mode.
The sensitivity displayed on the status
screen and LCD panel and in the
viewfinder changes.
Press the ® button to set the sensitivity to
[AUTO].
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L Take your finger off the ISO button and rear e-dial (;,,_,,¢).
The sensitivity is set.