1

Do all TTL systems calculate the same exposure?

Three options for example:

  1. Nikon SB-910 flashes, you can do TTL with them.
  2. You can use PocketWizard Flex/Minis for TTL flash
  3. You can use Profoto AirRemote TTL-N to do TTL

My question is do all these produce the same exposure? or do we have also good TTL and bad TTL? For example does a Profoto B1 in its own TTL mode will give me the exact same quality of picture if I use the same B1 but put a PocketWizard TTL on it?

3

I think you're confused as to what constitutes "a TTL system". Triggers merely relay the information from the camera to the flash. Whether you're using an AirRemote or a PocketWizard, if you're using Nikon's iTTL system, then you're getting iTTL information and calculations--it's still the same "TTL system" and the same exposure calculation is happening in the camera.

Because TTL systems are proprietary technology, how that exposure calculation is made is generally unknown, so Canon's eTTL may not be exactly the same as Nikon's iTTL, or Pentax's P-TTL, or any other maker's TTL system, so your question in a sense is relatively unanswerable. You could try measuring results to see, but that doesn't necessarily guarantee consistency.

In fact, TTL is not about shot-to-shot consistency. TTL is about speed. The basic function is that the camera tells the flash to throw out a "preflash" burst at a known power-level/brightness. This preflash is metered, and based on the results of the metering, the flash power is then automatically adjusted for the main burst.

Because this is meter-based exposure setting, just like all the other auto-exposure functions in the camera, the exposure will change based upon the scene's metering changing. And this is why most professional studio shooters shun it and prefer M mode on their lights. Just as you use aperture-priority for speed/convenience, you use TTL for speed/convenience, and whether it's the camera's shooting mode or the flash's power mode, you use M mode for precision and consistency shot-to-shot.

TTL is more useful for the on-camera flash photographer who is shooting an event and having to move rapidly in and out of different lighting conditions, who has to capture specific moments in a reactive way (a kiss, a wave, an unexpected "moment"), where split seconds count, and perhaps precision of flash exposure is of less importance to "getting the shot".

3

I think the answer is a "maybe". You're talking about different hardware running different software. There will be many occasions where the different combinations of hardware and software will reach the same conclusion and give you the same result, but there are times where there's a more borderline condition that will yield a different result.

But, I have to say, who cares? You're going to get a result regardless of what combination of hardware you use. How would you say what's "right" and what's "wrong"?

I could light a scene perfectly well with manual flash. It's possible the hardware may save you some time, but experience will reduce that edge.

  • 1
    Yeah, I have to say, in my experience, once you have more than one light, you don't want TTL anyway. – mattdm Jul 24 '15 at 20:45
  • Joe McNally is doing light 10 lights with TTL :D but still we can change their power separately in different groups – Brandon Jul 24 '15 at 23:49
3

Since "TTL" (system) in your original question is really about "thru the lens" the software the system uses to transmit that information really has no bearing on the exposure value (fstop shutter ISO ) calculation. What language the hardware talks is not changing the message. If the camera wants f8, and you ask a PW you get f8...if you ask another TTL trigger, you still get F8...

SO...truthfully...this has little to do with picture quality...and more to do with exchanging the settings (information) between devices.

Charlie

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