Canon EOS Rebel T4i 18-135mm IS STM Lens Kit Instruction Manual page 217

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 E-TTL
II (auto flash exposure), two flashes will be fired: Once when you
press the shutter button completely, and once immediately before
the exposure ends. Also, with shutter speeds faster than 1/30 sec.,
1st curtain synchronization will automatically take effect.
If an external Speedlite is attached, you can also select [Hi-speed]
(e). For details, refer to the Speedlite's instruction manual.
Wireless functions
When performing optical transmission wireless flash shooting using
the master function of the built-in flash, see "Wireless Flash
Photography" on page 225. When performing wireless flash
shooting with radio or optical transmission using the master function
of the external Speedlite, refer to the Speedlite's instruction manual.
Flash exposure compensation
See "Flash Exposure Compensation" on page 118.
Flash mode
You can select the flash mode to suit your desired flash shooting.
[E-TTL II] is the standard mode of
EX-series Speedlites for automatic
flash shooting.
[Manual flash] is for advanced users
who want to set the [Flash output]
(1/1 to 1/128) themselves.
Regarding other flash modes, refer to
your Speedlite's instruction manual.
3 Setting the FlashN
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