Basic help: How does Flash exposure compensation works

[Viggo]

In general ettl is ingenious to quickly dial in an ambient/object ratio. Not that the "correct" exposure at fec 0 often looks over-flashed, at least to me - you might want to start at -1 to get less broken shots. Raising shadows with fill flash in postprocessing is doable, but trying a over-flashed shot is a pita.

But even i set camera exposure to +1 and FEC to -3. Flash fires strongly resulting to not fire on the next shot..

The downside of ettl is that it relies on your camera's metering, so it's a good idea do be able to do both - ettl and m flash if you shoot the same or similar scenes again.

Note that there are two situations where ettl hurts:
[list type=decimal]
[*]To get *maximum* flash output, set the flash to m and full power. This way you don't waste power on the pre-flash which will only result in the camera telling the flash "dial to 11" anyway. The same applies if you want to do as many consecutive flashes as possible on lower output
[*]Automatic flash power might not be able to get the *minimal* flash output. If your subject is too near, and zooming the flash out, fanning and whatnot still over-flashes try to use m flash at min. power setting.
[/list]

Last not least, flash is indeed one of the subjects where reading a book or good website is really a good idea - unlike a lot of other photography stuff you can simply figure out reading/posting some forum threads and trial & error.

Are you sure the pre-flash is disabled in M mode? When I have used speedlites with for example the Quadra, there's an option to ignore a certain number of pre flashes to make the speedlite and the Quadra sync properly. I had loads of tries where the flash would trigger the Quadra too soon, and had to find the number of preflashes so they wouldn't trigger the quadra, and the speedlite was never in ETTL, nor was it triggered by the ST-E3, but the Elinchrom Skyports.

Correct me if I'm wrong

 

[Marsu42]

Are you sure the pre-flash is disabled in M mode?

Um, I was until you wrote it still triggered you optical slaves, but this could be a simply sync problem :-o ... why would the flash want to pre-flash in m mode?

Actually I read the method to max the flash power in a smart book, so I never really checked. But I hope pdb as "Mr. Flash" will give a definite answer on this w/o me checking...

 

[privatebydesign]

The problem is that there's now way (that I know of) to find out what power level(s) ettl chose, there's no tag in the image file or log on the flashes... so quickly switching m <-> ettl is always tricky. Or are there any tricks available?

No there isn't, the ETTL power levels, and the algorithms that drive them, are proprietary. Maybe you should get ML to look into it

 

[privatebydesign]

Are you sure the pre-flash is disabled in M mode? When I have used speedlites with for example the Quadra, there's an option to ignore a certain number of pre flashes to make the speedlite and the Quadra sync properly. I had loads of tries where the flash would trigger the Quadra too soon, and had to find the number of preflashes so they wouldn't trigger the quadra, and the speedlite was never in ETTL, nor was it triggered by the ST-E3, but the Elinchrom Skyports.

Correct me if I'm wrong

I am 100% certain that one on camera, or ETTL cord mounted, flash not set as a Master via flash menu or body menu does not emit a pre-flash. Any Canon flash used remotely set to M, not as a Master or Slave, just in M and triggered via third party triggers will not emit a pre-flash.

If your flash or body is set to have the on camera (or cord) flash act as a controller then it will emit a series of pre-flashes even in M.

I use PCB Einsteins with a 600EX-RT setup, I often use the 90EX as a dumb optical trigger for the Einsteins with it set to M and lowest power, there is no pre-flash, this is easily demonstrated by putting it in second curtain sync and a long exposure.

If I set the 600's to optical triggering, or even ETTL with the Radio, then I will get pre-flashes, if they are all set to M mode in Radio there are no pre-flashes.

In a mixed manufacturer flash setup RT and M still has no pre-flash.
On camera as a regular flash in M has no pre-flash.

In optical triggering mode Canon triggering always has a pre-flash, even in flash M mode, if your on camera or ETTL corded flash or body is set to controller mode it will pre-flash.

Lots of combinations but you can work with remote Canon flashes and not have pre-flashes.

 

[Marsu42]

Maybe you should get ML to look into it

I already did! Two problems here:
[list type=decimal]
[*]the ml devs don't care about flash (and they mostly shoot live view...) so the flash part is not fully reverse engineered.
[*]the flash firmware seems to be an "plug-in part" from another Canon dev team that is shared amongst cameras. The result is that the flash props are kind of dodgy and are not accessible all the time with external flashes. I'm still working on at least triggering two pre-set fec levels with a button...
[/list]

At least ML unlocks fec levels above and below the standard Canon +-3 for internal pop-up and some external flashes, unfortunately my 600rt is not among them.

 

[Viggo]

Are you sure the pre-flash is disabled in M mode? When I have used speedlites with for example the Quadra, there's an option to ignore a certain number of pre flashes to make the speedlite and the Quadra sync properly. I had loads of tries where the flash would trigger the Quadra too soon, and had to find the number of preflashes so they wouldn't trigger the quadra, and the speedlite was never in ETTL, nor was it triggered by the ST-E3, but the Elinchrom Skyports.

Correct me if I'm wrong

I am 100% certain that one on camera, or ETTL cord mounted, flash not set as a Master via flash menu or body menu does not emit a pre-flash. Any Canon flash used remotely set to M, not as a Master or Slave, just in M and triggered via third party triggers will not emit a pre-flash.

If your flash or body is set to have the on camera (or cord) flash act as a controller then it will emit a series of pre-flashes even in M.

I use PCB Einsteins with a 600EX-RT setup, I often use the 90EX as a dumb optical trigger for the Einsteins with it set to M and lowest power, there is no pre-flash, this is easily demonstrated by putting it in second curtain sync and a long exposure.

If I set the 600's to optical triggering, or even ETTL with the Radio, then I will get pre-flashes, if they are all set to M mode in Radio there are no pre-flashes.

In a mixed manufacturer flash setup RT and M still has no pre-flash.
On camera as a regular flash in M has no pre-flash.

In optical triggering mode Canon triggering always has a pre-flash, even in flash M mode, if your on camera or ETTL corded flash or body is set to controller mode it will pre-flash.

Lots of combinations but you can work with remote Canon flashes and not have pre-flashes.

Okay, thanks Have no idea what was up with my Quadra/speedlite system if it wasn't preflash. But they're both long gone so who cares ;D