Canon Speedlite 320EX - Manual (Page 30)

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With high-speed sync (FP flash), the flash can synchronize with all
shutter speeds. This is convenient when you want to use aperture
priority for fill-flash portraits.
1
Select [Shutter sync.].
Select [Shutter sync.] and press
<0>.
2
Set [Hi-speed].
Set [Hi-speed] and press <0>.
X High-speed sync is set.
High-speed SyncN
Check that is lit in the viewfinder.
If you set a shutter speed that is equal to or slower than the camera’s
maximum flash sync speed, will not be displayed in the viewfinder.
With high-speed sync, the faster the shutter speed, the shorter the
effective flash range will be.
To return to normal flash firing, set [Shutter sync.] to [1st curtain].
COP
Y