If I take a picture with an on-camera flash, or a single speedlight, my understanding is that the camera/flash pre-fire and then automatically select the approrpriate power level to properly expose the shot.

When you use two (or more) speedlights in a system (like CLS for instance), how do they automatically determine how much power each should use?

I can imagine plenty of cases where you would want one to be stronger than the other, would you have to set the power levels manually then?

I don't have any experience (yet) with off-camera flash, so I apologize if this is a stupid question, or if I have misunderstood how something like CLS is supposed to work.

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    Just to clarify something that might help things make more sense: the "power levels" don't adjust brightness, they adjust duration. TTL exposure control also adjusts duration. If one flash is set to full power and another to 1/2 power using the manual power settings, but the TTL decided that the total exposure should be less than the half-power duration, your settings would have no effect at all. During pre-flash, each group is fired at a known (low) level, and that is metered. It then consults the group settings to determine the actual duration required. – user2719 Oct 15 '11 at 14:08

In CLS, you organize your flashes into up to three groups (A, B and C; this is not the same thing as control channel). Each group is treated like a single flash. For groups set up to operate in TTL mode, during preflash, each group is metered separately (by camera, or by master flash in non-TTL auto mode), required power level is determined and during exposure each group is fired separately. You can mix up groups firing in TTL and groups firing in manual power.

On master flash, you can choose manual power setting or compensation level for each group and compensation level for the whole setup.

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  • As a user of a different brand, I gotta say this is a killer feature that makes me jealous. – Please Read My Profile Oct 15 '11 at 13:11
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    Just to note, I believe Canon with the 580EX supports similar functionality to Nikon's CLS...three groups and unlimited flashes, triggered by preflash. – jrista Oct 16 '11 at 17:34
  • So, if each group is metered separately in TTL mode, is the "required power level" set independently of the other groups? Wouldn't then the combined power of all groups be too much? – seanmc Oct 16 '11 at 20:20
  • And, if you want one group to be less powerful than another group, it seems you would do that manually using the "compensation level", or by taking the flash out of TTL mode and setting the power manually for each group? Then, you would need to meter (or take some test shots) to get your camera settings setup correctly? – seanmc Oct 16 '11 at 20:22
  • @seanmc yes, the groups are metered independently. Normally, you don't arrange two groups peaking light in same area; rather use on group as key light(s), one group as fill light(s), one group for background - so the groups actually make sense. TTL compensation is the easiest way to adjust relative power of groups, while manual mode excels in repeatability. TTL tries to magically get your flashes set up according to your camera settings, but testing might still give you some insight on what to tweak. – Imre Oct 17 '11 at 7:30

My speedlight transmitter has a ratio function. Ratio can be set between A and B flashes and the still be used in ttl. There is also a c channel which I guess you could use for hair light.

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  • Could you explain further about whether all Nikon Speedlight transmitters work this way? Or if you don't know, at least list what you have? – Please Read My Profile Nov 7 '12 at 15:31

The TTL groups are metered individually by the commander. The commander sets their power levels so that all groups are equal at the subject (regardless of their distances or their modifiers, etc). Normally one light is group A and one light is group B (to meter them individually). But both equal is flat light, and for best lighting results, we should set the fill flash compensated to be at least one stop down, -1 EV (lighting ratio). Much better light and gradient tones.

Yes all the flashes combining can be too much. 2X the light combining is one stop overexposed. The commander meters individually, and does not know how they add, so for safety, the commander appears to always set the exposure one stop low (underexposed). If setting fill lower, you will like it much better if you routinely start with +1 stop flash compensation.

And since you want fill to be -1 EV, the easy way to do this is to set the main light group to +1 EV, and leave the fill light group at 0 EV, so then you have both +1 compensation, and fill -1 EV relatively. For tweaks, the camera Flash Compensation still adds to both of these compensation values in the commander menu. If you want more, see http://www.scantips.com/lights/awl.html

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