I love narrow depth of field head shots. This one was recently taken of Lynn Tao, the CEO and founder of Juniper (http://www.getjuniper.com). She needed a head shot for some fund raising and we offered to take a couple of shots in our home studio before I gave birth (any day now).
Narrow depth of field lets you focus in on the eyes and lets the other elements drift off. The lighting setup was pretty straight forward and shown below. I used a softbox a bit off-axis exposed at f/8.0 as the key light, then used a shoot-through umbrella exposed at f/5.6 as the fill light, and then two strip boxes shooting down as hair lights exposed at f/5.6. These are pretty strong hair lights, which gave a nice shine to her hair.
One of the challenges with shooting with lighting is that you can’t shoot arbitrarily fast, typical sync-speeds with cameras are 1/160s to 1/250s. Thus to get a narrow depth of field you either have to turn down the lights very low, but that requires shutting out ambient light or you have to come up with a way to reduce the amount of light getting into the camera. I typically use variable neutral density (ND) filters to reduce the amount of light by some sizeable amount. The ND filter I used in this shot was 55mm filter that reduces light by a factor of 2 to 400, i.e. 1 stop to 8.6 stops. You can lose some sharpness with ND filters if they aren’t high quality. This one fortunately is a good one.
Another interesting feature about narrow depth of field head shots is that there is a minimal aperture you should ever use. If you frame a portrait head shot, it will be about 60cm tall. You always want to have about 3.5cm in focus. Now if you change focal length, keeping the aperture number fixed and keeping the framing of the shot, the hyperfocal distance will change, but the distance to the shot divided by the hyperfocal distance stays fixed. For a full frame camera, this works out to an aperture number of N=2.2, while for a APS-C crop sensor it works out to be N=1.6. I’ve attached a slide showing how the math works out.