Gear Talk > Lighting

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

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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:

--- Quote from: Steven_urwin on August 09, 2013, 10:29:46 AM ---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?

--- End quote ---

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:

--- Quote from: Frodo on August 09, 2013, 12:52:00 PM ---The downside is the pre-flash can startle birds prior to the actual flash - yes they react that fast.

--- End quote ---

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:

--- Quote from: Frodo on August 09, 2013, 12:52:00 PM ---Yep, Marsu nailed it. 

--- End quote ---

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.

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