On one of the forums I hang out on, there was a post asking for advice about how to take a candle-lit portrait of several children all at once. One of the responses linked to this site, which says:
If you mix outside light sources, be they tungsten bulbs or strobes with amber gels, you will lose the legitimacy of the scene. An image like this tends to look more set up and fake when you deviate from your original idea.
The problem was, though, that there are three children - and they’re just not going to stay still long enough to get a really long exposure. So I got to thinking, I bet you could do this with strobes…and the fact that the other site said that’ll never work appealed to my sense of adventure. :-)
As you may know, Denver is pretty much shut down, and we’re all snowed in…so I had limited resources for this. No one else is here, nor could they get here, so my mannequin head (which I call “Angelina” :-) was going to have to suffice. And I could only find one place in my house I could get things completely dark during the day today, and that was a bathroom. So forgive the primitive arrangements!
I first photographed the scene using candlelight only - this is what I got:

Nice and moody, pretty much what I was going for. This was shot at ISO 200, f/2.8, 1/4 sec, from a tripod. Any movement (subject or camera) whatsoever would blur the entire image.
Next it was time to try to simulate this with a strobe. I measured the candlelight to be somewhere around 2100K; a strobe with a full CTO gel is the closest I could get to that, in the neighborhood of 2800K, so that’s what I used.
I used a single strobe, using a 10 degree grid and the CTO gel, and positioned it so it would hit the face at roughly the same angle as the candlelight. Here’s a shot of the setup (like I said, primitive I know!):

Unfortunately, I couldn’t get the strobe further away from the subject, and I was on minimum power…so I was shooting at f/22. This required a 1/4 sec exposure to bring in the flame sufficiently; the difference, though, is that only the flame is being exposed for that 1/4 second. The subject is only illuminated with the strobe, for a very short duration - which means that if the subject is moving around, like those kids are inevitably going to, you’re still going to be fine.
In the end, this was the final result:

It’s close, in mood, to the candle-lit shot, although not perfect. Getting the strobe a little further away, or dialing down the power, would allow a faster shutter speed and/or showing more ambient light at the top of the candle from the flame. Generally, though, this is close - the light is falling off in generally the same pattern, color temperature looks roughly the same.
So don’t get me wrong…ambient light is great, and strobes aren’t for every shot. But in this case, where difficult subjects might be involved, using strobe could make the difference between taking home a good shot, and sifting through lots of “ok” shots looking for one that’s acceptable.