TTL flash, what's the big deal about TTL?

bambino · Mar 28, 2013

Ok, reading many forums everybody agrees being it a TTL flash it is so nice because it calculates the amount of light needed according to what the camera measures through the lens.

But, I have many questions here, Im confused, please be patient, Im so ignorant:

1)Do I understand correctly TTL concept? I understand TTL flash is as if the camera's sensor measures the light through the lens then it computes the correct amount of light needed to have the "right" exposure and then the camera "tells" the flash "you know flash, Im pointing at that subject which seems to be so far away and I have these light measure and we need a bit more light, can you help me?" then the flash repies "ok, camera, I can help you, just let me adjust my power and I deliver as much as you need but keep in mind Im blind and I do as you tell me, you are my eyes and if you tell me you are under an underexposing situation this much or that much I will fire my power accordingly".

2)If the flash decides how powerful it fires according to what the camera measures (it is TTL, through the lens, right?) then wouldn't it be that overexposure will often be the result. Lets say we are in fully manual M mode and we try to set light meter to the right middle of the rule (no underexpose, no overexpose) then we set Aperture and Shutter speed so now we have "perfect" exposure, right in the middle of the rule. Then if this is perfect exposure and the flash fires, I would expect an overexposure all the time!! Right? So TTL is doing a disservice, right?

3)If I want to do some off camera flash shots and I have my model looking at me then I have these remote trigger that allows TTL remotely. Then I point my camera to my subject. Camera reads light, measures iit an tells the flash "this is the measure, lets do the job, flash". But it happens I want to create some sort of glamour or beauty light that's the very reason Im using flash off camera. And the flash fires according to what the camera measures through the lens. But the camera has one measure according to its position facing the model while from the point of view of the flash, it is another measure because flash is facing the model from a totally different angle, right? Then the flash would fire thinking it is facing the model from the same angle. Then, the wrong lighting would result, right? So TTL is doing a disservice, right?

What Im getting wrong?

Sorry, and thanks!!

OLaA · Mar 28, 2013

I'm sure you have used your cell phone or point and shoot flash before. Exact same concept. Everything is communicating with each other. So when your in camera meter shows correct exposure it is also taking flash into account for that particular shot. It really depends what you want to do with flash that determines f you need TTL or not. Are you using it for compensation or artistic flare? If extra money is a concern manual flashes are relatively cheap and with practice and more experience you won't' need TTL.

Derrel · Mar 28, 2013

You have #1 pretty much correct. You are incorrect on your #2 postulation; overexposure typically does not occur...in the majority of cases, something fairly close to the 'correct' exposure results. #3??? Ummm, I dunno. What's the question? I was looking at the TV when I read that part of your post...

Buckster · Mar 28, 2013

Parts of the equation you may be missing:

With TTL, there's a pre-flash that the camera/flash brain uses to do the measuring, so it's not blindly looking at the scene as presented and doing the calculation. With that in mind, you'll want to meter on whatever it is you're lighting up with the flash, and a big part of that means choosing both your metering mode and the area of the frame to use for metering, depending on that metering mode.

Thus, for question 3, you need to actively choose to meter the part of your subject that will be lit by the flash to get it right, and you may have to get a couple of shots to dial it in even. But once you've done this a couple times with particular off-camera light positioning, you should have a pretty good idea where you need to be on settings with TTL for that kind of lighting.

In the end, most of us tend to use TTL more for "run and gun" type shooting, where we don't have the time to set up off camera lighting and pose our models, and instead use full manual in situations where we're going to take the time to set up off camera lighting and posed models.

cynicaster · Mar 28, 2013

OP, I think a big part you're missing is that the "brain" of modern TTL systems essentially considers a scene to be made up of two exposures: one for the subject, and one for the background/ambient.

It's hard to notice because it happens so quickly, but as Buckster mentioned, the system will fire a pre-flash as a reference that is used to calculate the amount of required flash power. Different manufacturers use their own proprietary voodoo, but the gist of it is as follows:

- you press the shutter button
- pre-flash fires, illuminates the scene, camera reads the result
- camera compares the exposure of the frame without flash to the exposure with the pre-flash. It does this in an attempt to figure out what the subject is--the subject will be illuminated by the pre-flash, whereas the background will be less affected, if at all.
- Once the camera takes a crack at guessing the subject, it meters the flash to "properly" expose that portion of the frame
- the main flash fires to expose the subject, and the background is exposed based on the usual shutter speed, aperture, and ISO

The systems even take a focus distance reading from the lens to help it guess the correct subject.

The manual-only zealots like to shun stuff like this but I think it really is a handy system. When you're moving around into all different lighting environments (say, at a social function), the TTL is handy because you can just keep shooting without constantly tinkering. It's not really necessary for posed studio type shots because the lighting environment is consistent and controlled, meaning you can dial in manual settings and leave them alone.

EDIT: just want to add, if you don't trust the camera to guess the correct subject, you can use flash exposure lock (FEL). You place your centerpoint on the subject, press the FEL button to activate a pre-flash and set the flash power, recompose the shot, and hit the shutter button.

hirejn · Mar 28, 2013

I have an article about TTL flash on my site if you wish to read it. You're making it pretty complicated but it really isn't. Forget about the distance stuff and where the flash is positioned. The flash will put out enough light to render the subject 18% gray. That's what it does every time it fires. That means it will overfill a dark subject and underfill a white subject. The camera also reads the entire scene when in matrix mode, so if the scene is dark, the camera will ask more from the flash and you'll get overexposure. If 75% of the scene is bright but the subject is dark, the flash may underfill the subject. So TTL flash changes depending on what's in the scene, just like matrix metering. The position doesn't matter. No matter where you put the light or how you point it, it will throw out enough light to render the subject 18% gray.

So why is this system desirable? Because if you understand how it works, it enables you to quickly get outstanding images without having to go through the steps of metering the light or worrying about distance to subject, bouncing or diffusers. But since it's TTL, pros put a lot of effort into understanding compensation. Some pros use TTL exclusively, and some choose to mix it with manual flash or just go manual, where they meter the light falling on the scene to avoid the camera's meter being fooled.

amolitor · Mar 28, 2013

Oh man, that's weird. Apparently you can't just measure light reflected from the sensor, so you have to use this pre-flash stuff with digital. That sucks. With film they just measured the light bouncing off the film and quenched the flash at the right time.

You can probably do a lot more clever stuff with the pre-flash though.