Flash weirdness

I got a strange one today. We finally smacked ourselves in the head when we figured out what it was, but hopefully someone else can get something out of it.

A friend of mine bought a JTL Versalight J-160, 160 Watt Second Monolight Strobe {adorama} and was trying to do a portrait.

The equipment:

7D + 24-105
Yongnuo YN-565EX ETTL Speedlite mounted on camera set to -1.5 power bounced off of ceiling.
JTL set to 1/16th power, optical slave.

1. With JTL off - just the Yonguno:
Exposure: Manual - 1/200th, f/5.6, ISO 800 -- Good exposure some shadows.
2. Turned JTL on, added a reflector to the side of the subject.
Exposure: Manual - 1/200th, f/5.6, ISO 800 -- black frame.
Checked to make sure the JTL was firing from optical slave. It was.
Used meter to ensure exposure values.
3. Reduced shutter speed to 1/40th, some exposure. Increased power on the JTL, some more exposure, still way under exposed. Increased FEC, black frame.
4. Turned JTL off -- Over exposed. Increased Shutter Speed to 1/160th, good exposure.
5. Turned JTL on - Black frame.
6. Switched out 7D with 1D Mark IV - repeated above - same result.
7. Switched out Yongnuo with 580EXII same result.

Anyone care to take a guess on what we did wrong for about an hour? Going how can we be adding more light and the frame be darker?

DO TELL...

Somehow the ETTL system was reading the optical slave and shutting off the sensor. Cannot figure out why, but that's what was causing the dark frames. Whenever the optical would fire, the sensor would shut off.

The Yongnuo being ETTL too, was passing on the same information to the camera.

Now why this doesn't happen with Alien Bees, White Lightnings, or any of the other half a dozen other optical slaves that I have used I have no idea, but this $80 strobe causes this.

Moral of the story, if you have a non-ETTL flash anywhere in your exposure turn ETTL completely off.

I've noticed playing around with a dumb slave flash (PZ-5000 AF) that when the E-TTL pre-flash fires to set exposure, it triggers the optical slave flash. E-TTL sees the light from the dumb flash and sets exposure accordingly (underexposes). The higher the power on the slave, the darker the actual E-TTL exposure. If power is set low on the slave, it will fire with the pre-flash and the real exposure. If power is turned up on the slave, it will only fire once with the pre-flash and won't recycle fast enough to fire again to take the picture.

Well according to the one page info sheet on the dumb slave it has pre-flash detection but who knows maybe it wasn't working as intended.

Just a suggestion. Have you checked that the speedlite was indeed in manual mode instead of ETTL?

It wasn't. I read it that way, too, but '-1.5 power' probably means EC, not manual. The pre-flash triggered the slave, the ETTL saw a ton of light and decided there was no flash needed.

EDIT: ANY time you have a black frame with optical flash, the first thing to think of is that you are in ETTL. Once it happens enough, you'll think of this automatically. Guess how I know? It's initially a bit maddening to see the remote flashes going off and there's no exposure. Once you get it, it becomes a bit humorous.

Yeah the -1.5 EC on the back of the flash was the clue that all of us missed. We should have realized that meant that we're still in ETTL and not "manual power settings".

Even running Canon's wireless system in manual, a pre-flash is used to instruct the slaves about power output before the real exposure occurs.