In certain TTL situations, especially when shooting in full sun or over long distances, the normal pre-flash is simply not enough for the camera to see. When Pre-Flash Boost is engaged, you get a pre-flash that is two stops brighter. This will approximately double the distance that the camera can make sense of the pre-flash TTL information or help compensate for the reduced output caused by a diffuser over the flash or a bounced flash.
Local Pre-Flash Boost Mode is for a transmitting radio, either MiniTT1 or FlexTT5 for Canon. It sets when Pre-Flash Boost is engaged for the flash on camera.
Remote Pre-Flash Boost Mode is for a receiving FlexTT5 only. It sets when Pre-Flash Boost is engaged for the flash attached to a receiving FlexTT5's hot shoe. Setting this mode on a FlexTT5 used as a transmitter will not engage Pre-Flash Boost for other FlexTT5 radios used as remote receivers. It must be set on each remote FlexTT5 individually.
- Disable Pre-Flash Boost
- Pre-Flash Boost will not be engaged and default Canon pre-flash levels are used. This is the default setting.
- Auto Pre-Flash Boost
- Pre-Flash Boost is engaged automatically when your Speedlite's flash head is angled in any direction other than the standard 90º straight-ahead position.
- Force Pre-Flash Boost
- Turns Pre-Flash Boost on full-time regardless of flash head position.
This mode is compatible with the following Canon Speedlites only: 580EX, 580EX II, 430EX, 430EX II, 270EX
This mode is not compatible with the following Canon Speedlites and will be automatically disabled when these flashes are detected in the shoe of a MiniTT1 or FlexTT5: 220EX, 550EX, 420EX.
Pre-Flash Boost is also not compatible with 3rd party flashes like the Nissin or Metz flashes for Canon.
Pre-Flash Boost for Nikon works for remote Speedlights on FlexTT5 radios only and is not used for a Speedlight on a transmitting radio. It is engaged automatically.
You can control how much "boost" is used by adjusting the flash output level/EV controls on the back of the remote flash. This allows you to increase the pre-flash boost beyond the default value for subjects farther away, and decrease the pre-flash boost for subjects very close to the flash.
A Speedlight set to TTL and 0.0 will use a 2 stops brighter pre-flash. If you increase EV, more Pre-Flash Boost will be used. For example, if you set EV to +0.3 then your pre-flash will be 2.3 stops brighter. If you decrease your EV then you will reduce the amount of pre-flash boost that is used. Set it to -2.0 to use native pre-flash levels.
The effective range of Pre-Flash Boost adjustment is from -3.0 to 0.0. You can use higher settings up to +1.0 but it may become less effective.
The amount of pre-flash boost compensation set on the remote flashes will not affect the final exposure.