Canon EOS 70D Instruction Manual page 197

Built-in Flash Settings
Flash mode
Shutter synchronization
If [2nd curtain] is set, the flash will fire right before the shutter closes.
When this is combined with a slow shutter speed, you can create a trail
of light such as from car headlights at night. With second-curtain
synchronization, two flashes will be fired: once when you press the
shutter button completely, and once immediately before the exposure
ends.
When using second-curtain synchronization, set the shutter speed to 1/25
sec. or slower. If the shutter speed is 1/30 sec. or faster, first-curtain
synchronization will be used automatically even if [2nd curtain] is set.
Normally, set this to [E-TTL II]. This
enables autoexposure shooting with the
built-in flash.
To set the flash output level manually,
select [Manual flash].
Select [2flash output], then set the
flash output level to within 1/1 - 1/128
(1/3-stop increments) before shooting.
Normally, set this to [1st curtain] so that
the flash fires immediately after the
exposure starts.
3 Setting the FlashN
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