Using flash to darken your background
Understanding exposure - exposure triangle) and how it controls timing and depth of field. Now were going to completely confuse you and throw in a fourth element; flash.
In a previous post we learned how to use fill light in high contrast scenes. When using flash for fill we are simply taking our existing exposure and adding light to the dark areas. The nice thing about using flash is that we can control the way the camera captures ambient light just by using the power of the flash unit to compensate our exposure.
One simple technique to illustrate this is the "invisible black background" technique as illustrated in the photo at right. The results are very dramatic. and the interesting thing is that this was all done in broad daylight out in the open.
I want to take this time to mention that this technique works best when the light is not glaring down creating hard shadows. You need to place the model in the shade and you should try to get a background that is fairly neutral in tone (no bright and dark spots showing).
You can also see that the light is coming from behind and on camera left. This creates some dark shadows on her face but we don't have to worry about that right now. We will be taking care of that later. At this time we just want to concentrate on the background. We want to take that nicely exposed background and get it so that it looks black. To do that we have to purposely under expose the background.
The second step is to shut you aperture just to the point that the background goes dark. Depending on the amount of ambient light that can be anywhere from f/8 to f/11 or more. Here's where you have to take a picture, preview it and make another adjustment as needed. Don't worry that you also can't see your model. Again, we are just setting our exposure for our background.
The reason I would rather move the flash closer over turning up the power is two-fold. By bringing the flash close in we can work at smaller power levels with the flash. That saves battery and increases recycle time. The second benefit is that the closer the softbox is to the subject the softer the light. We get more wrap to our light being that close. You can see in this photo that Bri is about a foot from the light. If you still don't have enough light with this set up then go ahead and start adjusting your power level up on the flash until you start seeing the desired results.
If you have a reflector and an assistant you can work the reflector on the dark side for some creative fill light.
So to review; shutter speed to the max at which you can sync (1/200 to 1/250) and aperture closed until the background turns black (f/8 or more). Set the flash at half power on manual to start with the softbox (umbrella, brolly box, etc) as close to your model as you can.