Canon PowerShot SX10 IS User Manual page 105

103
The maximum shutter speed with synchronized flash is 1/500
second*. Therefore, even if an aperture value is preset, the
aperture value may automatically be reset to match the
synchronized flash speed.
* When using an external flash, see pp. 258, 259.
Aperture Value Display
The larger the aperture value, the smaller the lens opening,
allowing less light to enter the camera.
F2.8 F3.2 F3.5 F4.0 F4.5 F5.0 F5.6 F5.7 F6.3 F7.1 F8.0
You can change the combinations of aperture values and
shutter speeds without altering the exposure (p. 132).