Originally Posted by gilp
I think that a 1200w/s flash may be a bit low power in bright backlit days, I use the ProFoto D4 ring light at 2400w/s and using the very valid technique discribed earlier, I sometimes need to had an external light source such as a hmi1200 daylight par.
the sun is a hard light to beat, thank god for iso50 !!
Actually 1200ws is more than enough. That would give us a guide number of somewhere around 450 at iso 100. By the sunny 16 rule, we would have f16 at 1/100th, but we can go up to 1/200th and still be in our sync speed. So that buys us a stop. You could then change to f22 which would buy another stop. So now we've reduced the effect of the sun by 2 stops. Now change to f32 and we have bought 3 stops reduction which will be quite dark for the background.
So, will the 1200 WS flash at GN 450 work at f32? Let's check. Divide 32 into 450 and you get about 14'. So if we put the flash at 14' the model will be well exposed and we will have a dark background. Suppose we use a less powerful flash with a GN of 320. Then you simply put the flash at 10'. But we could move the flash even closer, so we could go with a flash with even less power. But, the more flash power you have the more flexibility, such as lighting a larger area.