Canon Speedlite 430EX III-RT - Manual (Page 26)

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With high-speed sync, you can shoot with a flash even at shutter
speeds that exceed the maximum flash sync speed. This is convenient
when you want to shoot in the aperture-priority AE <
f > mode (open
aperture) with background blur in locations such as outdoors in daylight.
1
Press < 8 >.
2
Select the symbol in the
illustration.
Turn < 9 > to select the symbol
shown in the illustration, then press
<
8 >.
3
Select < c >.
Turn < 9 > to select < c >, then press
< 8 >.
Check that < F > is lit in the
viewfinder, then shoot.
c High-speed Sync
N
With high-speed sync, the faster the shutter speed, the lower the guide
number will be. You can check the effective flash range on the LCD panel.
When the shutter speed is less than or equal to the maximum flash sync
speed, <
F > is not displayed in the viewfinder.
To return to normal flash shooting, select < 2 > (first-curtain sync) in
step 3. (< 2 > will not be displayed on the LCD panel after you perform
this setting.)