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I just bought a radio trigger for my flash. I was just wondering if I understood TTL correctly, as I move the flash around and changes the distance does TTL automatically try to compensate (assuming it has the power to do so)?

Likewise, does it automatically adjust the flash exposure as I turn the head? If I tried to bounce the flash to the wall behind me, it will try to compensate and keep the exposure constant? Granted, the direction of the light will be different but the camera will at least keep the exposure constant based on the 18% rule?

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Assuming the radio trigger supports E-TTL II and it is the mode selected, the answer to all of you questions is "Yes." –  Michael Clark May 8 '13 at 7:11

2 Answers 2

up vote 8 down vote accepted

Yes. This is an advantage of through-the-lens metering, as the effect of the flash is judged based the light that, well, actually comes through the lens. It doesn't need to know anything fancy about angles; it just measures the effect on exposure. (on modern systems, using a low-powered "preflash" to test, and calculating from that).

This can be fooled by reflective surfaces, but in practice I find (and I've heard others also observe) that it's actually more accurate bounced than with the flash pointed straight ahead (probably that reflection issue).

Of course, this will only work if your radio trigger understands your camera's TTL system. Some do, but most don't.

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I will add to this by saying that for Canon, distance information can be gathered if the lens supports it, and if your flash is E-TTL II. Details on which lenses support this here: photonotes.org/articles/eos-flash/#distancedata –  dpollitt May 7 '13 at 22:52
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@dpollitt Originally implemented by Minolta, for photo trivia buffs. :) But of course that won't work with off-camera flash. –  mattdm May 7 '13 at 22:57
    
But why would the distance information help? There would only be one "correct" way to expose given a certain distance, so how would having that information help? –  erotsppa May 7 '13 at 23:44
    
It helps the camera decide if a surface is highly reflective or not. That prevents it from underexposing most of a subject who is, for example, wearing a jacket with a few reflective stripes on it. –  Michael Clark May 8 '13 at 7:08

Just to make sure we're clear. TTL flash doesn't adjust specifically because of or when you adjust your flashes. What happens is that when you go to take a photo, there are actually several flashes in rapid succession (so fast they look like 1).

If you are using optical sync, there is a control flash to tell the flashes to all pre-flash and configure their lighting levels. Then the camera meters the preflash, calculates the appropriate power levels and sends another control flash to tell the flashes what power to use.

The exposure then occurs with all the flashes firing according to the adjustment in total power that the camera used TTL to make.

You still need to do things like adjust the balance between flash groups to be the balance you want. I don't think any systems independently meter each flash group to ensure the balance of each group being recorded matches your ratio, it just controls the relative flash intensities.

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