Canon EOS 80D - Manual (Page 238)

3 Setting the FlashN
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 Flash mode
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.
 Shutter synchronization
Normally, set this to [1st curtain] so that
the flash fires immediately after the
exposure starts.
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 a more natural feel.
When second-curtain synchronization is set together with [E-TTL II], the
flash will be fired twice in a row: once when you press the shutter button
completely and once right before the end of the exposure.
Built-in Flash Settings
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 applied automatically even if [2nd curtain] is set.