Short comings of TTL flash, why the need for E-TTL

[Steven_urwin]

Hi everyone,

I was just looking at the theory side of flashes (very clever little bits of kit!) And as far as my understanding goes, with TTL, the flash will just push out light, until the sensor in the camera tells it to stop, i.e the correct exposure has been produced.

So the question, why did the need for E-TTL, with the pre-flash system come about? In what situation does E-TTL work, where TTL does not?

Thanks for your help, it's just bugging me, and I thought that someone here might know

Steve

 

[Marsu42]

So the question, why did the need for E-TTL, with the pre-flash system come about? In what situation does E-TTL work, where TTL does not?

E-TTL2 works in the ambient light, calculating the correct exposure by comparing the scene with just ambient & ambient+pre-flash, plus it takes the subject distance as reported by the lens into account. The result is an ingenious system where you can nail most exposures in varying light and distance and set the subject/ambient ratio with a turn of a dial.

 

[Frodo]

Yep, Marsu nailed it. The downside is the pre-flash can startle birds prior to the actual flash - yes they react that fast. In this situation I use manual flash, but TTL would be better. Otherwise, E-TTL is better.

 

[Marsu42]

The downside is the pre-flash can startle birds prior to the actual flash - yes they react that fast.

Not just birds, also some people seem to be "fast blinkers" and are notorious for having (semi-)closed eyes in the shot. And if really want to admire reaction time, ettl flash a rattlesnake, you can see the animal react faster than you can think it'd be possible.

Btw one other downside of ettl is that the pre-flash also triggers dumb optical flash triggers :-\ ... that's the origin of the dilemma what remote trigger system to invest in (IR or Canon or 3rd party radio).

 

[rumorzmonger]

Yep, Marsu nailed it.

Nope... not even close.

When TTL flash metering was developed, back in the film days (also known as "the good old days"...) the meter read the amount of light being reflected off the film surface during the exposure, to determine when enough light had reached the film to provide the "correct" exposure.

When digital cameras were introduced, it was discovered that the surface of the imaging sensor was too reflective compared to film, rendering TTL flash metering unreliable. E-TTL was the workaround developed by Canon (and similar solutions were developed by other companies) to reliably (or at least, more consistently) determine flash metering from a suitable reference surface (usually the front of the shutter curtain) before exposure.

 

[Steven_urwin]

So the question, why did the need for E-TTL, with the pre-flash system come about? In what situation does E-TTL work, where TTL does not?

E-TTL2 works in the ambient light, calculating the correct exposure by comparing the scene with just ambient & ambient+pre-flash, plus it takes the subject distance as reported by the lens into account. The result is an ingenious system where you can nail most exposures in varying light and distance and set the subject/ambient ratio with a turn of a dial.

This seems to be the answer I'm looking for, thank you. So just to give one example where TTL flash might fail is with a back lighting situation, as the flash might stop far too early, as the sensor would 'see' that enough light has come in through the lens, without knowing that the flash hasn't yet put out enough light, to light the subject. Is this correct?

 

[Frodo]

Yep, Marsu nailed it.

Nope... not even close.

When TTL flash metering was developed, back in the film days (also known as "the good old days"...) the meter read the amount of light being reflected off the film surface during the exposure, to determine when enough light had reached the film to provide the "correct" exposure.

When digital cameras were introduced, it was discovered that the surface of the imaging sensor was too reflective compared to film, rendering TTL flash metering unreliable. E-TTL was the workaround developed by Canon (and similar solutions were developed by other companies) to reliably (or at least, more consistently) determine flash metering from a suitable reference surface (usually the front of the shutter curtain) before exposure.

Well, both of you are right. Marsu's description of ETTL was correct. Your description of TTL is also correct. I wonder if sensor reflectiveness is the reason why Canon went down this route. In the days of film, I remember being frustrated that Olympus had an excellent TTL flash system and Canon never developed one for film. This frustrated me as a macro photographer, having to manually use flash with pre-calculated exposures depending on distance / extension. Were the patents for TTL so tight that Canon could not use it?

I would have thought that the reflectiveness of the digital sensor would have led to a technical workaround. Indeed, would you even need to measure light reflected off the sensor, rather than measure flash directly from the sensor? It seems to me that even if this were possible, that ETTL2, with the balancing of ambient and flash lighting, is a more elegant solution.

 

[Marsu42]

Yep, Marsu nailed it.

Nope... not even close.

I answered as I understood the op's question, not as a request for a history lesson but as "What happens if I put my speedlite in ttl mode" which for example is what happens if you use a cheap flash cord.

So just to give one example where TTL flash might fail is with a back lighting situation, as the flash might stop far too early, as the sensor would 'see' that enough light has come in through the lens, without knowing that the flash hasn't yet put out enough light, to light the subject. Is this correct?

Seems correct to me, though I never shot ttl and in practice ettl implementations vary across camera models. The problem here is the same as with camera metering: the computer doesn't know what flash/ambient looks "natural" to you and in what you'd like properly exposed in high contrast, so I most of the time end up doing 1/3-1 flash ec plus some camera ec (or shoot camera manual with flash ettl). On my 60d, I usually use -1/3 flash ec and in Av +1/3 camera ec combined with flash gels to gain a more subtle flash look.

 

[MarkusXXX]

I would have thought that the reflectiveness of the digital sensor would have led to a technical workaround. Indeed, would you even need to measure light reflected off the sensor, rather than measure flash directly from the sensor?

If you read the sensor, then you've taken the picture. Maybe there could be special pixels that can be read independently but if there are only a few there would be aliasing problems and with many of them the image quality would suffer.