+1 for flash in M mode. AV-TV is dead weight when you need to control ambient/flash exposure.
It is, could you elaborate? You control ambient via ec, imho Av is very handy when lighting changes on every other shot, esp. if you're able to select a min. shutter speed (newer cameras or Magic Lantern).but I will say that Av is much more effective on recent bodies where you can specify a minimum shutter speed
Did I mention Magic Lantern does this for the rest of us ("ML Auto ISO"), providing the level of control like the newer and expensive cameras (min/max iso, min. shutter in av, min. aperture in tv, ec on m) ... but you wouldn't know about programming your camera, would you, mr. 1dx :-p ?
For all the M mode fanatics: The newest ml builds have a "autoexpo function", you can define your own aperture curve and then some, imho this a terrific feature: http://www.magiclantern.fm/forum/index.php?topic=7208.0
ISO and aperture will affect both the ambient and flash exposure. The only control that affects just ambient is shutter speed. Lets take a indoor dance floor at a reception for example and your bouncing flash around.
So if your in Av... Your letting the camera decide how much ambient light will be in the exposure. A pain in the butt if light is constantly changing then you'll end up with a Coalminers cave and/or swirly lights at the cameras whim because it can be decieved by a bright spotlight or something else.
If you use Tv, then you control the ambient exposure but your DOF will be all over the place.
Now ETTL II does a good job at controlling how much flash to let out and not burning the subject white, but it has no control how much ambient light will be mixed in. You can try -1 expo comp and +1 flash comp to get something ok, but it still gets in the damn way if the meter catches a bight light source. I found it a frustrating experience to keep dialing in comp instead of just setting it once and use the flash ETTL.
Now outdoors overcast, you can get away with Av and flash because the light stays the same but you still give the ambient control to the camera....