Canon POWERSHOT SX150 IS User Manual page 97

You can set the aperture to a value at which you want to shoot. The camera
will set the appropriate shutter speed for the aperture value you set.
AM stands for "Aperture value", which refers to the size of the aperture
opening inside the lens.
;; Setthe modedial to AV.
Adjust the setting.
Turn the _i_ii_dial to set the aperture value,
Shoot.
Available Aperture Values
F3.4, F3.5, F4.0, F4.5, F5.0, F5.6, F6.3, F7.1, F8.0
Depending on the zoom position, some aperture values may not be available.
If the shutter speed appears in orange when the shutter button is pressed halfway, _
the scene has exceeded the correct exposure limits. Adjust the aperture value
until the shutter speed appears in white. Safety shift can also be used (see below).
In TV and AV modes, if you press the MENU button to display the _ tab, and
set [Safety Shift] to [On], the shutter speed or aperture value is automatically
adjusted for the correct exposure when it cannot otherwise be obtained.
Safety Shift is disabled when the flash is enabled.
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