How does that work? Doesn't that mean that the camera is controlling the flash intensity and duration, even though you're in Manual mode? I infer that there is a separate camera setting that controls whether the camera flash is in Manual mode or automatic? So, if the camera is in M but the flash is still in A, then despite your manual settings for A, S, and ISO, the camera will still light the flash if it thinks it needs to?
I had assumed that putting the camera in M would mean that the flash was also in M mode and you would have to manually set it if you wanted it to go off when you snap a picture.
Note: I'm talking about the camera's built-in flash because I think with a fiber optic cable, the camera's built-in flash controls the external strobe as well, right? Assuming the external strobe is in TTL mode, that is. So, whatever the answer is here, it's the same answer whether you are using an external strobe (in TTL) or not, right?
That depends!. There are 3 different things involved in the process, each controlled separately:
1) the camera - f/stop, shutter speed, ISO in manual mode. Variations of control in the other modes..
2) the built in flash - depending on your camera, it may be able to be set to only fire when needed, fire full all the time, only fire fractional power, etc.
3) the external strobe - it can be set to TTL where the on/off mimics what the camera flash is doing as seen through the fiber optic cable. The strobe has no idea what's going on, it's just doing what it's told. You can use the the rotary dial to tweak the light from the strobe. Or it can be set to manual mode, where the light from the camera through the fiber optic cable is just a trigger for the strobe. It then fires based on the settings on the rotary dial. If your camera uses a pre-flash to make it's adjustments, you have to use the 2 lightning bolt setting on the strobe so it knows to ignore the 1st flash. Otherwise the strobe will fire on the pre-flash and won't be able to recycle quickly enough to fire when the shutter opens. conversely, if your camera doesn't use a pre-flash, and you tell the strobe it does, the strobe will ignore the flash and wait for the 2nd, which never comes. Ideally, when using the strobe in manual mode, you should set the camera flash (if capable) to fire all the time at it's minimal power setting. That way, you get enough light through the fiber optic cable to fire the strobe, but conserve battery power that would otherwise be wasted firing the camera flash at a high power.