My favorite simple lighting setup.
It is, in fact, a very simple cross light setup using a shoe mount flashgun as the key and the sun to light everything else. I principal is pretty simple if you take a second to think about it.
Have you ever read your cameras manual? If you have, there was probably a page or two talking about keeping the sun behind the camera in order to prevent shadows on the faces of your subjects. While this is generally a good idea when taking snapshots of the family at the scenic lookout, it does not mean that it is not useful to place the sun behind your subject (out of the frame) from time to time.
There are two ways to look at this setup, traditionally the strongest light source is referred to as the "key" light. In most cases, if you are shooting outdoors, the sun is the strongest light source by far. In this case though, the placement of the sun behind the subject means that it is not able to fill the role of the key light and so we must use our own, in this case an sb-28. The problem now is that there is no way that our dinky little flash is going to make enough light to compete with the sun. There are several solutions to this problem. First you can shoot early in the morning or late in the afternoon. These times are often referred to as "Golden Hours" because the sun casts a pleasing warmer light when it is near the horizon. This is probably when you want to be shooting a portrait anyway as the warm light yields pleasing skin tones. The second part of of the solution is it to make sure you are shooting at your maximum sync speed. If you do not know what your maximum sync speed is then it is time to sit back down with that camera manual and learn about it. (on most cameras it is around 1/250s ) The reason for this is that faster shutter speed will kill the maximum amount of ambient light (Sun light) while leaving the light from your flash alone. This should allow you a pretty good balance between the ambient and your flash pop.
The rest of the setup is easy! Set up your flash and stand so that it is at about a 45 degree angle to you and 180 degrees from the sun (cross light). I like to use a shoot-through umbrella to that the light from the flash stays nice and soft, if you were carrying around a studio in your pocket this light would be a strobe with a softbox. Before you turn on that flash pop off a couple frames and stop down your lens to just under the proper exposure for the background. Then turn on your flash and slowly up the the power level until the shadows are gone from the near side of your subject. From there you can experiment with the contrast of the exposure of your subject (controlled by the flash) and the exposure of the background (controlled by the sun) until you find something you like.
The beauty of this setup is that you only need one light (pack light!) and the effect can be subtle to dramatic depending on how hard you push the lighting ratio. Another cool thing is that, if you add a reflector 90 degrees to the flash (camera right in the above example), then the sun effectively becomes a simple rim light and you can work with an even greater ratio of flash to sun.
All of the gear needed for this setup can easily fit in a smallish camera bag and there is really no excuse for not traveling with this gear any time that you bring a camera with you, this can really bridge the gap between an available light portrait and a more elaborate studio style look.
My favorite simple lighting setup.