Skirball said:
Auto ISO, in Av, with eTTL... At that point you might as well use the green square.
Av generally Does Not Do What You Want when using speedlights. Make sure you have read & undersand this:
http://photonotes.org/articles/eos-flash/index2.html#flashmodes
I haven't used the 6D with flashes much yet, but when I was shooting lots of flash stuff with the 40D, the way I preferred to work in low light with flash was:
- shoot in M mode
- set ISO as high as tolerable (maybe 1000 or so)
- user a wide-aperture lens & set shutter speed as low as tolerable (can be 1/30th for easy stuff with an IS lens, but more like 1/100 for dancing etc.
- aperture one click off of wide-open
[*]set flash up for bounce off ceiling or 45° diffusion with a softbox, and use a color filter on it to get it close to the color of ambient light. Leave flash in auto e-ttl2.
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In M mode the flash becomes your 4th exposure parameter after ISO, aperture, & shutter, and it's automatic. Adjust flash exposure compensation to get ETTR where you want it.
This method augments the ambient light with the flash & gets very natural looking shots (if you get it right, many non-photographer people don't realize that they're flash photos). If you've got a decent amount of ambient light it also keeps the flash power down so that your batteries last longer & your speedlight recycles faster.
On the 6D I'd probably set the ISO to 6400 and be a little more conservative with the shutter speed, but so far I haven't needed flash as long as I stick to fast primes.
BTW I've got a little 270EXII that lives in the bag now, which compensates nicely for the lack of built-in on the 6D. I leave lithium batteries in it as they're as light as you can get & they don't fade.
