How much light does CNTL flash "spill" into scene?

Started Jan 10, 2013 | Discussions thread
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William Porter
William Porter Senior Member • Posts: 1,676
How much light does CNTL flash "spill" into scene?

So, using the Sony wireless flash system, with a flash on the camera acting as control or triggering flash, and a remote flash "out there" as the unit being triggered, the question arises, how much flash does the control contribute to the scene?

It's my understanding that the answer is supposed to be None. The on-camera control flash is supposed to act as a trigger only, and is not supposed to add its light to the scene.

Now I've heard a number of people in this forum and elsewhere say that this is basically how it works, but that there may be a little spillage from the scene if you're really close to what you're shooting. In other words, some posters admit that it can be a problem but insist that the amount of light added is small and that you have to be really in the subject's face for it to matter.

Neither answer jibed with my experience, so I decided to give it a quick test. Here's the executive summary. I may blog about this in the next day but I wanted to post here first to see if anybody can point out any stupid mistakes I may have made.

I did most of the testing with the A99, with an HVL-F43AM on the camera as control and an F58AM actually in the scene as the remote. I wanted the remote in the scene so I could really see how much light it was contributing – in order to distinguish that from the light contributed by the control. Needless to say I don't shoot this way normaly, actually, doing this had never occurred to me before.

I put the camera into Manual mode. Here's a baseline shot, without flash:


Now here's a shot with the control flash (F43) turned on. In this shot, flash exposure compensation set on camera to -3. The remote (F58) is sitting on the top shelf of the little bookshelf at the end of the hallway, with its sensor pointing up but its flash output pointing down so it's muffled. It's that bright, blown-out spot just past the mirror, top shelf. In this shot, the F43 control flash is pointing directly forward.

Same scene, with flash.

That's shot in ADI flash mode, but I got an identical result in P-TTL.

Now, here's what I got when I simply turned the control flash away from the scene (off to the right, so it was shooting into my kitchen):

Same scene, with flash — but with control flash pointed off to the right, away from the scene.

I took a bunch more shots but these tell the story, or so I think. The difference between the shot immediately above (#3) and the preceding shot (#2) is the contribution of the control flash. No, it's worse than that, because #1 is much darker on the right and left than #3, which indicates to me that some of the light from the control was bouncing back from my (rather open) kitchen and adding to the scene's illumination, even in #3.

Note that I'm NOT all that close to the subject. And the amount of light contributed is by no means trivial. For what it's worth, these shots were taken during the day time and there are a few windows, but it's a gray cloudy day here, the light in the house didn't change between shots, and the shots were all taken at pretty close to the same time. Not scientific but I was keeping that in mind.

I tried this with different settings on the camera but no matter what I did I got pretty much the same results. The whole gallery is here:

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Panasonic Lumix DMC-LX7 Olympus E-M1 Panasonic Lumix G 14mm F2.5 ASPH Olympus M.Zuiko Digital ED 40-150mm 1:4-5.6 R Olympus M.Zuiko Digital 45mm F1.8 +7 more
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