Canon EOS 70D Instruction Manual page 201

Flash exposure compensation
Flash exposure bracketing
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 [Second-curtain
synchronization] is set.
With an EX-series Speedlite not compatible with flash function settings,
you can only set the following: [Flash firing], [E-TTL II meter.], and
[Flash exposure compensation] under [External flash func. setting].
([Shutter synchronization] can also be set with certain EX-series
Speedlites.)
If flash exposure compensation is set with an external Speedlite, you
cannot set the flash exposure compensation with the camera (Quick
Control or External flash function settings). If it is set with both the
camera and external Speedlite, the Speedlite's setting overrides the
camera's.
The same setting as step 3 in "Flash
Exposure Compensation" on page 190
can be set.
For details, refer to the Speedlite's
instruction manual.
While the flash output is changed
automatically, three shots are taken.
For details, refer to the instruction
manual of a Speedlite compatible with
flash exposure bracketing.
3 Setting the FlashN
201