How does TTL flash work when there are multiple flashes?

Question

I've been watching a lot of deconstructing videos on how the light is setup. Often times there are 2-3 flashes at the studio. The photographer will say that they are all set to TTL. But how does that work?

Say one main light is on the person, one is shooting at an object he is holding, and one is lighting the backwall. How does the camera know which setting to put each flash at to arrive at the "correct" exposure?

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It seems most of the answers are focused on answering how TTL works. I know how TTL works and how preflash works.

What I don't understand is how TTL and preflash work when there are 3 flashes firing at the same time. Does the system simply make a global adjustments to all 3 flashes and hope for the best? Even with the ratio set, it still doesn't explain how the TTL value is obtained.

Answer 1

While the specifics are somewhat brand-dependent, this question has essentially been answered already in one of your follow-up questions.

Start with the following assumptions:

1. There is no magic involved; everything that happens will be as simple as it possibly can be and still work;

2. The system is not and cannot be foolproof; any sufficiently advanced fool can easily defeat it (a corollary is that the system can be "gamed" to advantage by anyone who knows the system);

3. Modern TTL systems, unlike the TTL-OTF systems used in the film era, do not measure the actual exposure while it is happening; and

4. In an optical multiflash system, no information can be transferred from flash (or IR controller) to flash without something flashing. (Radio systems, like the new Canon 600EX-RT/ST-E3-RT can do some additional trickery without flashing, but there is no evidence to suggest that the method of TTL metering is different.)

All of the remote flashes are fired at a known power level (1/32 power usually, but that's brand-dependent) at the same time, before the exposure, at the command of the "master" in the system. This will probably not result in a correct exposure; the metering system will decide how much the overall flash power needs to be adjusted (and in which direction) in order to achieve a "correct" exposure. This is the "0.0", uncompensated power.

If you have made no adjustments, either overall using flash exposure compensation (or, in Nikon's case, the combination of exposure compensation and flash exposure compensation) or using the group settings, that is the power level that will be relayed to all of the flashes in the system (all the flashes "listening" on the same channel) before the main firing signal. If you have set an overall compensation, then the "master power level" (the "0.0" value arrived at above) will be adjusted appropriately before being relayed.

If you are using groups set to different power levels, or the "ratio" feature that's sometimes available (particularly in macro systems), then the "master power level", adjusted for overall compensation, will again be adjusted in accordance with the settings you've used. If you have Group B set to +1.0, then all of the flashes in Group B will be told to fire at a power level that's twice as high as the compensated "0.0" value. If Group B is set to -1.0, of if Group B is on the "1" side of a 2:1 ratio setting, the flashes in Group B will be told to fire at half the power level.

It is, of course, possible for all of this to go horribly wrong, and the chances of it going badly increase with the number of flashes you use—if you don't understand how the system works for your camera and flash system. If you do understand the system, you can do an awful lot more than the limited number of groups and settings available may suggest.

You need to know what your camera is metering. You need to understand that a bare speedlight close to the thing it is illuminating will have a much greater impact on the metering exposure than a diffused speedlight at a greater distance. You need to understand that, say, a 580EX will put out more light at a given power level than a 430EX will (the same goes for the SB910 versus the SB700 and similar pairings), and that it's the relative power levels, not the absolute output, that the system is adjusting. And that two speedlights in one softbox or umbrella will put out twice as much light as one speedlight in another at the same power level (so that you can still get a 2:1 lighting ratio even when all three flashes are in the same group). And that you can gel individual lights down within a group as well to reduce the power of individual flashes without using up an extra channel.

Answer 2

For multi-flash TTL setups, the photographer first puts the flashes in groups and then adjusts the power between those groups by setting up power ratios. (Say for example that Group A should have 4 times the power of Group B, or a 4:1 ratio.) Prior to the pre-flash, the master flash communicates these ratios and the overall power level to be used to the flashes. The pre-flash is fired, the lighting measured and adjustments are made to the overall power level to adjust the exposure. This update is then communicated to the flashes and actual image capture then takes place.

This either happens wirelessly through a radio link or optically through a series of quick flash pulses that occur so fast that they all appear to be a single flash, despite potentially actually being a series of 20 to 30 flashes.

Answer 3

In modern TTL systems, camera sends digital commands to flashes (by means of optical, radio or wired signalling).

First, a preflash at minimum power before actual exposure is triggered and measured to calculate how many times the power of flashes should be increased in order to attain desired exposure. This power level is then communicated to slave flashes. Finally, during exposure, triggering signal is communicated.

In simplest setups, all the flashes fire at same power level, and intensity provided by an individual flash is adjusted by adjusting distance, angle or modifiers.

However, some systems (such as Canon's E-TTL or Nikon's iTTL) also allow photographer to divide flashes into groups (e.g. key, fill and hair lights) and set power ratios between those flash groups, or level compensations can be set for each group / individual flash. In such systems, several preflashes are used in rapid succession to meter each group separately, and also each group is sent their own power level. In case some flashes (or flash groups) have significant overlapping lit areas, photographer is expected to dial in corresponding negative compensation; but that kind of setups are rarely used anyway.

So, in the example, each of the three flashes would be metered using their own preflash, and accordingly assigned their own power level (such that the flash individually would provide correct exposure) by the TTL camera. If the photographer wants more emphasis on the person, he adjusts ratio towards that flash, so other flashes will be commanded weaker power levels.

Answer 4

What I don't understand is how TTL and preflash work when there are 3 flashes firing at the same time.

The answer may be somewhat dependent on the system that you're using. In Canon's system, and probably others, each flash group emits a separate preflash. The camera can then determine power levels for each group based on the total light needed for correct exposure and the flash ratio set by the photographer.

For example, if you set groups A and B to a 4:1 ratio, the camera will measure the ambient light, the light from group A, and the light from group B. It then sets the power levels such that you'll get a correct exposure with 4x more light coming from group A than from group B.'

Even with the ratio set, it still doesn't explain how the TTL value is obtained.

Again, each flash group is measured individually.

Answer 5

Ok, here is my simple explanation. I hope this answers the question. I use TTL with 2 units. I set my "B" unit so it's 2 stops less flash than "A" unit. I set that ratio using my on camera transmitter.

I would think that the "A" unit would dictate the overall exposure. As long as "A" is the correct exposure, then "B" would simply fall into place with the 2:1 ratio I set.

I would not think "B" would dictate the exposure at all and also wouldn't think it would independently have its own reading. That just makes sense. The way I describe it is just common sense and simple. There's got to be only ONE main unit (A) that allows the other unit to always remain fixed in its ratio (mine being 2 stops lower in output)

Another bit of info: when my rim light flash (B) is allowed to emit its pre-flash directly into the camera (not totally hidden from the camera's view), it WILL throw off my overall flash exposure a lot. As long as you keep any unit from direct pre-flash toward the camera, the B unit will remain fixed in its ratio, and A flash will remain as the dominant flash unit.

Answer 6

Since all flashes fire at the same time (triggered by a pre-flash or radio signal) the camera does the exact same exposure check it does without flash. TTL means through the lens, so it just looks at how much light hits the sensor.
There are different exposure measurements, like e. g. "global metering" (as I'll simply call it), which uses complicated algorithms, or the straight forward spot metering mode. But those surely are the same for TTL flash metering as for regular, non-flash exposure metering.

Answer 7

If the three flashes are in different groups, they pre-flash one by one and the power needed is determined for each flash individually.

If the flashes are all in the same group, the system does indeed make global adjustments to all of them at the same time. It does not hope for the best though. It has measured that it needs X times the intensity of the pre-flash for a correct exposure, so it will tell all flashes to fire at X times the intensity of the pre-flash. Now it has a correct exposure. How much light is coming from which flash is completely irrelevant. If you want to change the effect of the individual flashes, you have to put them in separate groups, or move them closer or further from the subject.