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When using Nikon D7000 with an off-camera SB600 (which cannot be used as commander) in TTL mode, the built-in flash must be opened to make it work (to serve as a commander), even if this built-in flash is set to off.

This built-in flash does not trigger when taking a photo, but does preflash. Why?

Searching for an explanation, I found that this is required to measure something. But what?

  • A shutter speed in aperture mode? In this case, the speed will be completely wrong, since during preflash, the scene will be lighter than when taking photo, since in this last case, the built-in flash will not trigger.
  • Built-in flash intensity? But why do we need it, since the built-in flash will not be used while taking photo?
  • Remote flash? Again, the measure will be wrong, for the same reason as in the first point.

What I'm missing?

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If your on-camera flash is affecting your exposure, you can use one of theses 'shields':- nikonusa.com/Nikon-Products/Product/Flash-Couplers/4905/… –  rapscalli Jan 9 '11 at 17:55
    
Man, that is one ugly contraption. Too bad they don't have something that would just slide over the built-in flash. –  mattdm Jan 10 '11 at 5:37
    
Hmmmmm.... amasci.com/amateur/irgoggl.html#diy –  mattdm Jan 10 '11 at 5:41
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2 Answers

up vote 5 down vote accepted

The popup flash is being used as the master to trigger SB600 which, when used this way, is actually being fired by the preflash of the popup. Basically, the SB600 detects the light of the master and when it does, it fires.

Edit:

As a note, there are radio based triggers that can do this without the need for the popup. Nikon is very well supported in this realm with one of the best being PocketWizard though there are much cheaper (and less reliable) options such as the Cactus v4 trigger set.

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Ok, so I misunderstood how remote flashes work. I thought there is an infrared signal used, and this infrared signal is sent by the DSLR body. Thanks for pointing me out in the right direction. –  MainMa Jan 9 '11 at 12:52
1  
@MainMa: They can also work in IR, like when using Canon's ST-E2 trigger. But it turns out that in most cameras there already is a device that can be used to generate pulses of light -- their built-in flash. Adding another IR source probably woundn't be worth it. –  che Jan 9 '11 at 16:08
    
In TTL mode, it's more complicated than that. This answer really shouldn't be voted up so high..... –  mattdm Jan 9 '11 at 17:58
    
@mattdm: Well, it's still a correct answer. Popup flash fires because it's used as a signal for the SB600. –  che Jan 9 '11 at 21:20
3  
@John Cavan - The slave flash isn't detecting the light of the master; it's detecting the digital signal which happens to be carried in the light. This is an important distinction because in "dumb" slave mode flashes do respond to just the light itself. Additionally, there's the more pedantic distinction between the preflash (where the signal strength is measured and which happens before the shutter is open) and the control signal (which actually happens at the beginning of the exposure) -- that is, it's literally wrong to say that the SB600 is being fired by the preflash. –  mattdm Jan 9 '11 at 22:12
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So, actually, the preflash is a two-way dialog between the control flash (in this case, built in to the D7000) and the remote units. There's a reverse-engineered )and several years old, so possibly slightly out of date) explanation of the protocol by Alson van der Meulen. (The site is offline but archived.)

Basically, the control flash fires a minimal pulse, which actually triggers a reduced-power response from the remote flashes. The TTL system in the camera measures this response, and uses it to calculate the right power levels for each. (This may or may not include the control flash.)

Then, the controller flashes again with a reduced-power series of pulses which digitally encode instructions for the remotes, and in response every flash that's included in the exposure fires its main pulse.

All of this happens so quickly that it's impossible (or at least extremely difficult) to observe it unaided.

However, the last control flash actually happens with the shutter open, so even though it's at low power, if you have a wide aperture, or if a reflection of the camera appears in the scene, it can actually be seen. I assume this is a necessity in order to get the timing to work out.

(This works the same way for Pentax as well, although without separate control groups. I assume Canon and Olympus are basically the same but I don't know.)

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I have a D90 with a SB700 which is a similar setup. I wonder, if I would have a second SB700 (or 800 or 900) to use as a master instead of the popup flash, would the control flash also happen at exposure time? Or do they work differently? –  Luciano Oct 27 '11 at 18:43
    
The same, although if you can angle or turn the flash head you may be able to reduce annoying reflections from the controller. –  mattdm Oct 27 '11 at 18:59
    
See photo.stackexchange.com/questions/16702/… –  mattdm Oct 27 '11 at 19:00
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