I have a couple
TTL flashes (Nikon SB-910s) and also a a couple of PocketWizards but I see on the internet that there are triggers that support TTL and that confuses me.
If I am understanding correctly, TTL is dependent on the focal point of your lens, so if TTL flash is sitting on your camera they yes TTL has a meaning because the flash is in the same distance from the subject as the focal point of your lens but when we take the TTL flash off camera and put it like two meters away then to me TTL is meaningless.
If that assumption is correct then what do they even mean by Wireless Triggers that support TTL?
In that case do they just mean you can manually set the power and zoom of each remote flash without having to walk to them? So basically just setting zoom and power remotely is what they mean by TTL triggers?
I have a couple
TTL is dependent on the focal point of your lens
This is not the case. Some flashes have an automatic zoom, which automatically adjusts to the focal length of the lens.
so if TTL flash is sitting on your camera they yes TTL has a meaning because the flash is in the same distance from the subject as the focal point of your lens
No, not necessarily. You can angle the flash in pretty much any direction, even to fire the light behind you, in the opposite direction that the camera is facing.
TTL takes all that into consideration, because it measures the flash exposure through the lens, as Detlev describes it.
If you angle the flash in any direction, at least those that I own stop the automatic zoom adjustment, because as you say: it makes no sense.
TTL employs a pre-flash to calculate the power requirement for the main flash.
For that pre-flash it is irrelevant where the flash sits - if it is further away, less light reaches the target and the main flash is adjusted accordingly to output more power to obtain what "it" considers an appropriate exposure (taking into account any flash compensation as set by the user).
TTL simply means that the camera tells the flash to send out a "preflash" burst of light at a known brightness level that the camera can meter, so it can have a sense of how to set the flash power for the actual shot. Distance, focal point--it still just gets metered. Where the flash is doesn't really matter--the metering itself is still done from the camera.
However. TTL triggers aren't necessarily prized by off-camera flash shooters for supporting TTL function. TTL triggers communicate most of the full hotshoe electronic communication protocol--not just the sync signal, like manual triggers do. So, more of the on-camera function of an OEM flash can be used with a remote flash--like controlling the flash mode, or the manual power level or FEC adjustment with TTL remotely. HSS/FP can also be used. 2nd-curtain can be used. Adjusting settings on the flash remotely. These can be extraordinarily convenient, vs. having to walk up to each individual flash to adjust settings.
So, no, a TTL-capable radio flash trigger is not meaningless.