Canon 550D - Manual (Page 194)

3 Custom Function SettingsN
194
C.Fn-6 Highlight tone priority
0: Disable
1: Enable
Improves the highlight detail. The dynamic range is expanded from the
standard 18% gray to bright highlights. The gradation between the grays
and highlights becomes smoother.
C.Fn III: Autofocus/Drive
C.Fn-7 AF-assist beam firing
The AF-assist beam can be emitted by the camera’s built-in flash or by an
external, EOS-dedicated Speedlite.
0: Enable
1: Disable
The AF-assist beam is not emitted.
2: Enable external flash only
If an external, EOS-dedicated Speedlite is attached, it will emit the AF-
assist beam when necessary. The camera’s built-in flash will not fire the
AF-assist beam.
3: IR AF assist beam only
Among EOS-dedicated Speedlites, only those which have an infrared
AF-assist beam will be able to emit the beam. This prevents any
Speedlite which uses a series of small flashes (like the built-in flash)
from firing the AF-assist beam.
With setting 1, the [Disable] setting automatically takes effect for the
Auto Lighting Optimizer (p.103) and it cannot be changed.
With setting 1, noise may become slightly more pronounced than usual.
With setting 1, the settable ISO speed range will be 200 - 6400.
Also, the icon will be displayed on the LCD monitor and in the
viewfinder when highlight tone priority is enabled.
If the external, EOS-dedicated Speedlite’s [AF-assist beam firing] Custom
Function is set to [Disabled], the Speedlite will not emit the AF-assist beam
even if the camera’s C.Fn-7-0/2/3 is set.