A quick lesson in how I use camera flash to improve people pictures.
The first point of interest is "why" would I/you need to incorporate a flash for a portrait. The quick and short answer to that question is to improve a dull or uninteresting/uninspiring image.
How you would do that depends completely on the situation you are presented with. It would be difficult to illustrate that in a blog post, as a video would be more appropriate. However, I will do my best to give the reader some sort of understanding as to my approach.
Hypothetical situation (A)
You and your subject are in a room with a chair near a big bright window. The sun is streaming in, illuminating part of your subject's face. Since it is strong and contrasty, the lit side of her/his face is quite bright, and subsequently the shadow side is very dark. The unlit parts of the room are dark and very moody, almost creepy.
To lessen the creepy-factor, it would be wise to reduce the shadows as to bring up detail where there is none. (of course this is assuming you are not creating a scary portrait)
The key here is that we want to reduce the depth of the shadows, not eliminate them completely. We need shadows to show shape and form afterall.
First, shoot an image and look at your monitor. When you have the highlight side of the subject's face looking satisfactory, not too bright nor too dark, make note of your camera settings. You can either establish this using one of the camera's priority modes or in full manual. It doesn't really matter. What matters is that the bright, sun-lit side of the face is exposed well.
Now use your flash, mounted to the camera if necessary. Preferably operated off-camera using a trigger, cord, or other firing method. Let's assume it will be on camera. Turn the flash on and set to manual. I would advise that you swivel the flash head towards the wall that is nearest the shadow side of the face. Set the power setting low. For example 1/16th power to start. Take a shot. Look at your monitor and see if the shadows have brightened up to the point where you can clearly see details in the shadows.
Still too dark for your taste? Increase the power output by a ⅓ stop and shoot again. Keep doing this until the shadows are acceptable to you.
You could also do this with the flash in TTL mode. I would however, use the flash output compensation mode in this case as you test things. Have it set to a negative value and slowly increase it in order to achieve the shadow value you desire. Again, bouncing the flash off of the wall nearest the shadow side of the subject's face. Keep doing this until you get the image you desire.
Situation (B), outdoors in bright sun.
In the next situation, you might be outdoors on a sunny day with deep shadows present all around. I would use my flash off-camera if at all possible in order to put details in the shadows of a person's face and to add a bit of life to deep eyes.
Getting the camera off of camera is ideal, however if you can swivel the flash head in order to bounce the light off of a lightly coloured wall, shirt, vehicle etc. that will work as well. The key is to fill in deep shadows in order to bring out the details. If you are prepared or otherwise have an assistant who can carry equipment, you can incorporate a light modifier of some sort in order to make this work even better.
Exposing for the brighter part of the face comes first. This can be achieved using manual mode just as easily as shooting with your camera in TTL mode. Test it and see. If your subject has similar skin tone as yourself, hold your hand up into the same light situation as them, and photograph your hand without the flash incorporated. When the skin tones look correct, note your camera settings.
Now get your flash going and adjust the settings of the flash in order to give you a gentle bit of fill on the shadow side. If the sun is coming from the left in this example, also have your flash coming from the left. This will reduce the chance of creating weird shadows on the face. Play and see what looks best to your eye. Practice and practice again. Don't be afraid of flash. It won't hurt you or your subject.
Photo-Artist working a personal vision.
For those of us interested in better marketing techniques, get this book.