Canon EOS RP, Advanced User'S Manual, Page: 228 / 612

228
Setting the Flash Function
z
Shutter synchronization
Normally, set this to [First-curtain
synchronization] so that the flash fires
immediately after the exposure starts.
If [Second-curtain synchronization] is set, the flash will be fired 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 flash metering] , 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.
If [High-speed synchronization] is set, the flash can be used at all shutter
speeds. This is effective when you want to shoot with background blur (open
aperture) in locations such as outdoors in daylight.
o
When using second-curtain synchronization, set the shutter speed to
1/80 sec. or slower. If the shutter speed is 1/90 sec. or faster, first-curtain
synchronization will be applied automatically even if
[Second-curtain
synchronization]
is set.