OM-D E-M10 flash: 2nd-C vs SLOW2?

With 2nd.C

- ACC down, there is a pre flash.

- ACC up, there is a pre flash.

Right. No knob on the strobe is going to change what the camera is doing. But, if there is a pre-flash, the ACC should be up, to tell the strobe that the camera is going to do a pre-flash.

What I meant to say

With 2nd-C

- ACC down, Z-240 flashes twice

- ACC up, Z-240 flashes twice

I have tested this with two different Z-240's.

What I meant to say

 

With 2nd-C

- ACC down, Z-240 flashes twice

- ACC up, Z-240 flashes twice

I have tested this with two different Z-240's.

Okay. And neither option produces an image with a proper exposure?

I have noticed that my E-M10 flashes twice when in Aperture priority mode and the flash set to SLOW2. I believe whatever is happening is the same as what you are seeing. I have been trying to find an explanation for why it flashes twice but, so far, had no luck. I can only guess that the first flash is actually a "pre-flash" (i.e. before the shutter opens) used to determine the actual exposure values to be used, including the duration and intensity of the second flash.

If you use TTL the camera will pre-flash at first curtain and flash at second. In manual it will only flash at second. I don't find very useful