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

Started Jan 10, 2013 | Discussions thread
William Porter
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sybersitizen is right (again) — the ANSWER
In reply to sybersitizen, Jan 11, 2013

sybersitizen wrote:

BTW, the 'huge' amount of light you're seeing from the flash controller is because you're shooting at f/2.8 and ISO 800/1600. Don't do that if you don't want so much extra light in the image. Use normal flash parameters by lowering the ISO and/or closing down the lens.

I would make a bad testing engineer, as this thread proves conclusively. I've been hearing these conflicting claims for a year or two and wanted to see for myself. I opened the aperture up precisely because I wanted to call as much of the "slop" as possible. Bart7D asked if this problem occurred in real life, and I tried to test that — but that test (with my dog) was done with wide aperture and high ISO. Anyway, I posted here hoping for correction. Seemed like the right thing to do. But not everything that seems like a good idea, actually is.

In short, YOU ARE RIGHT.

It just boils down to a case of the old joke where the patient says, "Doctor, it hurts when I do this" and the doctor says, "Don't do that."

Here's a shot taken in my office, with basically no light other than that supplied by the two flashes. Before I hit the shutter I was sitting in the dark. Aperture is wide (f/3.2) but the ISO is down to 200.

I don't see any light spillage or slop here from the control flash. I would expect to see some light on the near side of the chest of drawers on the right if the control were having an impact and I don't.

I will note that I took this picture twice — with HSS enabled, and with HSS off, and I see no difference. You said that there would be no slop with HSS on, and only a little with it off, and I can't see the difference.

This explains several things to me, but I'll mention just one. It explains why I haven't had a hard time with this in my own limited multi-flash studio work: I almost always shoot a portrait at ISO 200 or ISO 100. I use higher ISOs a lot when shooting events like receptions, but I'm not using wireless flash there.

So is that it? I'll have to look in the users guides for the various flashes and see if they say anything like, "When in wireless flash mode, stop the aperture down and/or lower the ISO in order to reduce or eliminate the impact of light from the control." I doubt they do but I'll report back if I'm wrong about that.

I wish now that I had my old Pentax system here to try it. I'm right about one thing: the Pentax on-camera flash can act as control OR master. I'd add I don't remember ever hearing about light spillage in control mode. But I'm not sure I used my Pentax flashes much in mere control mode, now that I think about it.

One last question: Why do people keep talking about using a piece of film to cover the triggering flash?

Will

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