Canon 40D - Manual (Page 128)

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Whenever you set the power switch to <1/J> or <2>, the Self
Cleaning Sensor Unit operates (approx. 1 sec.) to automatically shake
off the dust on the front of the sensor. Normally, you need not be aware
of this operation. However, you can execute the sensor cleaning at
anytime as well as disable it.
1
Select [Sensor cleaning].
Under the [6] tab, select [Sensor
cleaning], then press <0>.
2
Select [Clean nowf].
Turn the <5> dial to select [Clean
nowf], then press <0>.
Select [OK], then press <0>.
X The screen will indicate that the
sensor is being cleaned. Although
there will be a shutter sound, a
picture is not taken.
In step 2, select [Auto cleaningf] and set it to [Disable].
X The sensor cleaning will no longer be executed when you set the
power switch to <1/J> or <2>.
3 Automatic Sensor Cleaning
Cleaning the Sensor Now
Disabling Automatic Sensor Cleaning
The user-executed cleaning takes about 2.5 sec. to complete.
For best results, do the sensor cleaning while the camera bottom is
placed on a table or other surface at a perpendicular angle.
Even if you repeat the sensor cleaning multiple times, the result will not
improve that much. Right after the sensor cleaning is finished, the
[Cleaning nowf] option will remain disabled temporarily.