jeffery wrote:@ prinothcat : Interesting to suggest ND filters. I have no experience with them, but from what I know, they reduce light by 1/2/4 stops? Would it only reduce light in the background in this case? I would have thought it'd take light out of the entire frame.
Yes you have that correct a full ND would drop the EV over the entire frame.. The ND graduated filter (ND grad), however, only darkens one half of the frame. They come in hard or soft transitions. A full frame style is available as well that goes full dark on one edge to clear on the other. I prefer to use square filters, in a Cokin holder that rotates freely in front of my lens so that I can adjust where the transition falls, simply by turning the holder and sliding the filter in and out of the holder.. In theory at least you could have lowered the exposure value for the top right corner with a judicious alignment of a Grad. As far as the actual filters go, Cokin are an inexpensive to learn how they work. The draw back of Cokin I have found is that they are not color neutral, for some reason they have a subtle red coloration. They do make a very useful holder however, and most every other maker has filters that fit that Cokin frame.
I'll also chime in on the fill flash idea, and suggest that a slow sync setting, or slow rear setting may help as well since the primary exposure is metered then the flash fires to make up for the (in your case) backlighting. These allow the metering system to make the best exposure it can off of the ambient light then provides the fill. Both should also allow the blur from the motion of her arm, helping to eliminate the strobed look.