When shooting TTL, the external strobe fires just at the level necessary to perfectly exposure the shot. This does not necessarily require any communication between the camera and the strobe, it just requires the strobe to mimic the firing pattern of the built in flash. When shooting non-TTL, i.e. manual, the strobe is just triggered by the camera’s built in flash and fires at the level to which it is set on the strobe itself.
Shooting TTL in the camera’s Auto modes is trivial, shooting manual in the camera’s Manual mode is trivial as well but sometimes you want it differently:
- Shooting TTL with the camera in manual mode saves you the hassle of choosing the strobe power. You decide the shutter speed and aperture (and thus the foreground vs background brightness) and the flash fires just at the level that nothing is over exposed (It could be under exposed if your settings are not right).
- Shooting Manual with the camera in auto mode is great to save battery life as the built in flash can be set to minimum power (all it does anyway is trigger the strobe). With clear housings this is a great advantage to eliminate backscatter from the built in flash.
The fact is TTL is not that reliable underwater and with time you may move to shooting manual anyway but I find that it helps a lot before you master that ability.
Some cameras allow choosing TTL or non-TTL in all modes, other limit TTL to Auto modes or non-TTL to manual modes, or both.
To learn more about TTL and see some examples of TTL in action click HERE.