Why does using my slave flash darken the image?

Question

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.]

Answer 1

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.

Answer 2

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.

Answer 3

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.