As someone who once shot
everything in the studio with continuous light (because there was no flash then) and who also lights TV commercials, perhaps I can add to this...
Back in the day, when we only had continuous lighting, it was acceptable - just as a 1956 Ford Popular was acceptable in 1956 - but the world has moved on. We no longer want to produce the rigid, stiff portraits of the pre-flash era, where people sat still during a long exposure and smiled grimly at the camera, we now notice if the pupils have all but disappeared because of the bright lights shining in the eyes and we now worry much more about the fire safety risks of using hot lighting.
Of course, lights don't have to be hot anymore, fluorescent lighting can be fine, but the fluorescent lights available at a price to suit the home user are pretty poor and don't even begin to compare with the ones used in TV studios, and are a joke compared with HMI lighting. Even the professional lights used in film and TV are extremely uncomfortable to work under, but at least most of the studio spaces are massive, with very good ventilation/extraction, and most of the people filmed are pros who are used to working under very bright lights, and they've been made up professionally too - very different from home studios.
But the biggest single problem with continuous lighting is that a lot of the lights can't be adjusted for power, and even those that can don't have a massive range of adjustment and as there is never enough power to start with there are even more power problems when the lights are turned down...
Add in the fact that there are limited or no light shapers/modifiers available and it should be clear why nearly everyone has moved over to flash. In fact, it's almost certainly true to say that only complete beginners normally buy continuous lighting, these are the people who don't understand the issues I've mentioned above, but mostly they move on very quickly, either giving up studio photography of moving to flash.
BTW, this reply from vaizki is 100% right
Some tips to experiment with if I may...
- Have her sit at a 45 degree angle with her right shoulder closer to the camera, preferably sitting lower so you can shoot from a bit higher angle
- Head turned towards the camera and then just a bit "over" towards the shoulder so she's facing a bit left of camera but lock eyes with the camera lens
- Place the SB900 well to the left for "short lighting" so that the side of her face that's NOT towards the camera is lit more. Fill in with another flash (camera's own in a pinch) to remove the harshness from shadows.
- Get the exposure time up to 1/160s to kill ambient light. You are now mixing flash and room lighting and it'll be a nightmare to get the white balance right. Flash exposure is not controlled by shutter speed, only by aperture.
- No sleeveless clothing in head & shoulder shots.. The skin of the arms detracts from the face. Try to choose clothing that blends in more with the background, that way the viewer will be focusing on her face.
Lots of things to try out but I think they'd be worth a shot.. or two