I was answering a question in a thread, but decided it was a little too off topic and thought it would be more useful in it's own thread:
do you have any reception shots, dancefloor etc that you can show me?? would love to see your results when shooting at 400iso with flash...and also your settings too??! cos at the moment i use ambient light alot cos i always find my flash gets rid of a lot of the ambient..! and i was thinking of buying a 7d, just for the higher iso range!
I'll try and find some... but that would require some digging to find. You can totally shoot at 400 if you are capable of lighting EVERYTHING. But to be honest, I actually prefer to shoot at 1600 (or 800) for two reasons... 1600 on my 40D looks fabulous, and it saves a whole ton of flash power and reduces recycle time. But my shots would look exactly the same whether at 400 or 1600 because I basically kill all ambient light. That's why high ISO is no longer critical.
Anyhow, here is an example I pulled from an Indian wedding I did...
The mehndi hall. Crappy light. It sucked, like most ambient light in almost all venues. I use a lot of flash because I know I can make much better light than what's available. And this place was no exception. Dim, diffuse, flat, even, and boring lighting.
1. This is what it looked like. ISO3200 F/2.8 1/50. 1/50 isn't a great speed for candid shooting.
2. So I put three lights into the scene. One on each side and one behind the seat for the bride. Whoa... now I'm at ISO400 f/4 1/160. The exposure now kills all ambient light from the scene.
3. I put my 40D on mehndi macro duty that night, so I actually shot with the two other bodies I brought... 1Ds and D700. Now I was shooting at ISO200 F/1.8 1/125 on my 1Ds. It allowed me to produce absolutely clean, crisp, and detailed photos with light that was SO much more interesting than the ambient.
4. Once people hit the dancefloor, it didn't matter how fast they moved. I was getting all the detail and crispness I could want. Flash gives me that. ISO12800 will NOT. I was shooting my D700 at ISO100 f/2 1/125 in the EVENING! booya!!
5. I took a couple shots with ambient light. This is with my D700 at ISO1600 F/2 1/125. It lacks the detail, colour, contrast, and crispness of a flash shot. One reason why I prefer flash is because higher colour temperature of flash bring out contrasts and colour better. Also, ambient lighting is almost always overhead, which causes raccoon eyes (two women standing) and overly bright noses (both girls in green) which I hate.
6. And then I took the same scene with flash. ISO100 F/2 1/125. So much cleaner, crisper, and so much more detail which you cannot see in web size, but makes a huge difference in album pages.
7. This was two days later. My favourite ultrawide dancefloor setup is my 40D and 10-22. As I was saying before, flash brings out a lot of detail and contrast. A well flashed 40D dancefloor shot at 1600 actually provides me more detail than my D700 does at 1600 using only available tungsten light. The combination of action freezing flash and the higher colour temp achieves this. The sensor of the 40D is certainly inferior but the environment I provide for it makes up for it. This is why I just do not care about high ISO performance. This was taken with my 40D at ISO1600 F/5.6 1/125. Yup, I use F/5.6 in dark places because flash allows me to. I manually set focus to about 1.5 metres at 10mm, and all I gotta do is hit the shutter and I get everything I want in focus. I don't touch the AF button for the rest of the night. I had very mixed results doing this on full frame at 16-35, having to go down to like F/10 and suffering with a combo of killing my flashes, increasing recycle time, and increasing noise by cranking ISO, and still getting mixed results.
The thing is, it's not that hard to learn to use flash. ANYBODY can do it. Even if I had a camera that could do a clean ISO51200 I would still be using flash at low ISO just because I can make light that is so much nicer than the ambient. Seriously, give it a whirl; you might surprise yourself. It's nowhere near as hard as people think it is.