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Why does my slave flash at full power actually seem to darken the image?

The camera (Lumix G1) is set to M-Mode, forced flash at -2, f6, 1/30s. I set my external flash (Nissin Di466 FT) to slave mode "S2" to ignore the first "measure" flash.

I do an experiment with three shots:

1) Slave flash off: Image is medium bright.

2) Slave flash +0.5: Image is brighter

3) Slave flash +1.5: Image is really dark with a red tone

How can this be? Even if the slave flash has missed the right point in time, image 3 should still have the same brightness like image 1, since the camera flash is still on. Am I missing something?

(iExposure is off, and ISO is set to 200).

[Edit: it turned out that on the Di466 "S2" is the mode that does NOT ignore the pre-flash. But I still do not understand the behavior.]

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3 Answers 3

up vote 4 down vote accepted

You guys are on the right track. Here's what's happening in each picture:

Slave flash off.

  • Your camera fires a pre-flash
  • The camera determines how much flash power it needs for proper exposure based on how bright the return flash is.
  • The shutter opens, and the camera fires its flash at the power level it just determined.

You get a medium-bright picture.

Slave flash at +0.5

  • Your camera fires a pre-flash, which triggers the slave flash.
  • The camera sees a very bright return flash, so it decides to use very low power on-camera flash for the exposure.
  • The shutter opens, and the camera fires its flash at a very low power, which triggers the slave flash again. At only +0.5 power, the slave flash still has enough charge to fire again immediately.

Although the on-camera flash was very dim, the slave is much brighter than is needed for proper exposure, so the picture is overexposed.

Slave flash at +1.5:

  • Your camera fires a pre-flash, which triggers the slave flash.
  • The camera sees a very bright return flash, so it decides to use very low power on-camera flash for the exposure.
  • The shutter opens, and the camera fires its flash at a very low power. However, the slave flash, at +1.5 power, has already used more than half of the charge in its capacitors, so it can't fire again immediately.

Since the on-camera flash is dim, and the slave doesn't fire while the shutter is open, this picture is darker, and only consists of ambient light. Since your camera thought the scene was going to be lit mostly by flash, it balanced for xenon light. The ambient light (which is probably tungsten or tungsten-colored CFLs) is a much lower color temperature than xenon, so your picture has a red tint.

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Thank you Evan, this makes perfectly sense. I was not aware that in S2-mode the slave flash might fire on the pre-flash AND the main-flash if it has enough power left in its capacitors. –  Stiefel Feb 17 '11 at 21:02
    
A simple experiment confirms your theory: in S2-mode with +0.5 power the slave needs twice as long to recharge than in S1-mode - which means the slave flash has fired twice. –  Stiefel Feb 17 '11 at 21:24

Your slave flash is not fully recharging before firing at the appropriate time and you're seeing the ambient light.

Here's how it works; when your slave is set to a lower output it's actually firing twice because it's 'seeing' the 'pre-flash' from your camera and firing on the 'pre-flash' and the actual flash from your camera.

When you crank up the juice on your slave it's firing on the 'pre-flash' and it doesn't have enough time to fully recharge before it's being asked to fire again (at the appropriate time).

So, the only thing that's left is the existing light in the room, likely an incandescent bulb which is casting a red/yellow cast on your image.

EDIT: As a very unscientific test, you could darken the room completely, slow your shutter speed to about 3 seconds and as soon as you hear the 'shutter' open fire the flash manually (at the higher setting). If that produces a brighter exposure then you know the flash wasn't firing when it should have.

EDIT 2: After some looking around, I'm under the impression that a flash mounted on the hot-shoe will disable the pre-flash. If you have a spare that you can crank down to it's minimum that may do the trick.

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He said he has it on S2 though, which will ignore the first flash. –  Daniel T. Feb 16 '11 at 2:23
    
From the OP: [Edit: it turned out that on the Di466 "S2" is the mode that does NOT ignore the pre-flash. But I still do not understand the behavior.] –  Ofeargall Feb 16 '11 at 3:09
1  
Thanks for your answer. I dont think that is the right answer. First, I am pretty sure the slave flash has recharged (its indicated by a green light). Second, I should see ambient PLUS the build in flash from my G1. I have another idea: maybe the slave flash fires during the pre-flash (Since it is set to S2) - right wenn the camera is doing some TTL measurements. Now the camera sees this bright flash and reduces the intensity of the second internal flash. This would explain the result, right? –  Stiefel Feb 16 '11 at 8:20
    
That's a definite possibility. When you disable both flash units, does the ambient give the image the red/yellow look you're speaking of in your post? You could try mounting the external unit on the hotshoe of the G1 and see if you get a more accurate exposure. Mounting on the hotshoe should disable the TTL preflash from what I've read. Or, you could try the other, less scientific method I mentioned before. Let me know what happens. Also, can you post a sample image? –  Ofeargall Feb 16 '11 at 15:38
    
@Stiefel, upon further consideration I'm not convinced that your slave is firing on the second flash after the TTL preflash. If, as you proposed, the camera read the slave during it's TTL preflash measuring, it still wouldn't change anything on the exposure or the slave since they're both set to manual. The camera 'may' reduce the built in strobe because of the scenario you suggested but the slave remains unaffected. –  Ofeargall Feb 16 '11 at 21:16

My guess is that, on number 3, your slave firing on the preflash is making the camera's meter think there is way too much light and no flash at all is needed, so all you're getting is ambient. Number 2 is a bit of a mystery though.

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