Canon EOS Rebel XTI - Manual (Page 111)

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When you turn the camera’s power switch on or off, the Self Cleaning
Sensor Unit operates to automatically remove dust adhering to the
imaging sensor. You normally need not pay attention to this feature.
However, if you want to activate this self-cleaning manually or disable it,
follow the instructions below.
Select [Sensor cleaning: Auto].
Under the [b] tab, select [Sensor
cleaning: Auto], then press <0>.
With [Clean now] selected, press
<0>. The self-cleaning will be
performed for about 1 sec., then the
menu will reappear.
With the screen above displayed,
press the <U> key to select [Set
up], then press <0>.
Press the <U> key to select
[Disable], then press <0>.
To minimize dust from entering the camera, note the following
countermeasures:
When changing lenses, do it in a place with minimal dust.
When storing the camera without a lens attached, be sure to attach
the body cap.
Clean the body cap before attaching it.
3 Automatic Sensor Cleaning
Manual self-cleaning
Disabling automatic sensor cleaning
For maximum effectiveness, set the camera upright horizontally (do not
tilt up or down) before executing [Clean now].
If [Clean now] is grayed out and cannot be selected, wait a while until it
is selectable.