All the discussion in this forum about the 5D3 in low light has me intrigued. I'm particularly interested in a 5D3 w/24-105 f4L IS as an upgrade path for a 7D w/17-55 f2.8 IS. (For low light candids, I'm often shooting at 1/30 second and find IS to be a must.)
I know that the 5D3 offers greater color depth and that full frame is typically sharper than crop. I also understand that the 5D3 offers about a 1.3 stop advantage in noise over the 7D. But, I'm comparing a crop body with a 2.8 lens to the full frame with a 4.0 lens and this noise advantage drops to about 1/3 of a stop.
Now the question. A 2.8 lens lets in more light than a 4.0, which is more light for the AF system to lock in. So which system can lock in on focus at lower light -- the 5D3 w/24-105 f4L IS or the 7D w/17-55 f2.8L IS?
Quite frankly, I'm not exactly sure where you'd expect a big improvement worth $3000+ with that approach. I'm still on my 5DII and don't own either the 7D more the MarkIII but the question of "low light performance" and high ISO stuff wouldn't cross my mind as a top priority even though I actually shoot candids in ambient light situations quite often.
Lens/sensor size choices to me are primarily a matter of a) which (real) focal length do I want to use and b) how important are options for narrow depth of field. And even though I own the 24-105 the IS is not really part of the equation. Yes, it'll help a bit for those shots where you're at 1/30 or even less. Will it get you quality outcomes? Probably not. Camera shake is only one problem here and when photographing people it only saves you to some degree - say it bumps a complete fail to a still usable snapshot which can of course be a good thing but is nothing I would want to count on a lot. Sorry, it's one of my pet peeves and I really don't like that everyone is now calling for IS-everything when that should really remain a feature for a) very long focal lengths and b) snapshot amateur cameras.
One more exception: the 24-105 I found to be a very capable concert photography lens where visible movement is actually desirable. Anything more formal? Eh, not really. Still prefer my 50 and 135 for low light candids.
The other thing is flash. I know some people don't like using it and there are of course circumstances where it's simply not an option. However, if possible I like using it even for those low light ambient candids. I put on half or full CTOs and try to keep as much of the ambient light as possible. With that even low(ish) shutter speeds give you good results - with the added benefit of catch lights which I like.
As far as higher ISO goes I find that anything up to 3200 easily works with these modern cameras and post processing gets rid of any issues that you may have. I personally don't mind a bit of grain and sometimes add film grain as opposed to cleaning things up.