Canon EOS 20D Instruction Manual page 145

C.Fn-14
0: Evaluative
Fully automatic flash photography for all conditions, from low light
to daylight fill-flash.
1: Average
The flash is averaged for the entire area covered by the flash.
Since automatic flash exposure compensation will not be
executed, you may have to set it yourself depending on the scene.
This also applies if you use FE lock.
C.Fn-15
0: 1st-curtain sync
1: 2nd-curtain sync
When a slow shutter speed is set, you can capture a light trail
following the subject. The flash fires right before the shutter closes.
This Custom Function can be used to obtain 2nd-curtain sync
effects even with EX-series Speedlites which do not have this
feature. If the EX-series Speedlite has this feature, it will override
this Custom Function.
When 2nd-curtain sync is used, a preflash will be fired for flash metering
control right after you press the shutter button completely. Remember that
the main flash will fire right before the shutter closes.
C.Fn-16
0: Disable
1: Enable
This works in the shutter-priority AE (Tv) and aperture-priority AE
(Av) modes. If the subject's brightness changes suddenly and the
current shutter speed or aperture becomes unsuitable, the shutter
speed or aperture is shifted automatically to obtain a suitable
exposure.
E-TTL II
Shutter curtain sync
Safety shift in AV or TV
3 Custom Function SettingsN
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