Pentax K-5 Operating Manual page 65

If the CMOS sensor becomes dirty or dusty, shadows may appear in the
image in certain situations such as when shooting against a white
background. This indicates that the CMOS sensor must be cleaned.
Dust adhering to the CMOS sensor is removed by applying ultrasonic
vibrations to the filter on the front surface of the CMOS sensor for
approximately one second.
1
Select [Dust Removal] in the [a_ Set-up 4] menu and press
the four-way controller ( • ).
The [Dust Removal] screen appears.
Press the
OK
button.
The Dust Removal function is activated.
Set [Start-up Action] to _ (On) to turn the
Dust Removal function on every time the
camera is turned on.
When Dust Removal is completed, the
camera returns to the [ak Set-up 4] menu.
D :
Start-up Action _,_