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See this tutorial of mine which explains portrait lighting basics: http://photo.nova.org/CluelessToCompetent/
The two ingredients needed are fill near the axis of the lens to provide the foundation of shadow detail, with a directional "key" light to create the 3D modeling of the face.
There are many possible angles to put the key light relative to a face but due to the recessed eyes and nose sticking out in the middle of them only a few are flattering to the subject — getting light into both eyes and creating a nose shadow falls over 1/2 the nose, modeling its shape it a realistic way.
For a full face pose a very flattering strategy it to simply raise the key light flash so it is directly over the camera in line with the nose hitting at a downward angle of about 30-45° to the eye line: high enough to model the shape of the cheeks, but no so high the brow shades the light from the eyes. A single flash raised on a bracket will produce that pattern very effectively and simply indoors and out...
Those are all single flash shots taken with a single flash on a bracket with my DIY diffuser:
The difference between the first and second shots is the amount of "spill fill" which occurred. In the first situation I wanted to isolate the subject from the distracting background and tried to minimize spill off the 10ft ceiling. For the second shot I wanted maximum "spill fill" off the ceiling so I stood on a chair and pressed the top of the diffuser to the ceiling producing a combination of a direct "key" component from the diffuser and a large "spill fill" source from the ceiling to open the shadows in the same way an overcast day reduces the scene range. The outdoor shot utilized the skylight as both "key" and "fill" before flash was added by the simple expedient of knowing how to pose my wife to the direction of the skylight before adding the flash. Then I just added the flash from the same downward flattering "butterfly" angle from the bracket and diffuser.
The thing those three shots have in common is that various means were used to control the fill in the shadows, but the fill wasn't really that critical because keeping the "key" lighting centered and above the head doesn't produce any unflattering shadows. That's why its a good idea to use a flash bracket if you are serious about producing flattering results with speedlights, especially in situations where only one flash can be used.
When using two flashes the off camera flash becomes the "key" light and the Master in the camera becomes the fill. The advantage of that is being able to precisely control both the lighting pattern on the face via the placement of the key light relative to the bridge of the nose (where the nose shadow originates) and light is needed in the eyes. So when placing the key light don't think in terms of where the light is relative to the camera or spot on the floor as in a lighting diagram, but where it is in space relative to the bridge of the nose of your subject.
Here's the scenario for creating what is called a "short" lit oblique view of a subject's face. Start by walking your light stand around to the side and behind the face of the subject until it is 45° from their nose. Since their nose is 45° to the camera that will put the light stand about 90° to the camera. Stand behind the light using the stand like the front sight of a gun and move it until you see the subject's face in a perfect oblique view where the > notch of the far side eye is seen, but the bridge if the nose isn't blocking your view of the far eye. What you will be doing is using your eyes like modeling lights. What you see from behind the key light is what will wind up highlighted in the portrait — you want light in both eyes so you need to see both eyes, but you don't want to light or see the far side (from the light) ear. It sounds complicated but with a bit of practice you'll be able to just look at the angle of light to the face and it will wind up where it needs to be 45° to the side of the nose.
Natural light comes from overhead most of the day so to make the flash modeling more natural you want the key to come from a downward direction of 45° also. So ideally your key flash winds up at a point in space which is 45°H and 45°V from the bridge of the nose. Then all you need to do is walk back around to the front, focus and shoot...
That was one of the first test shots I made with my DIY modifiers and my wife talking was oblivious to what I was doing. That's typical of a lot of candid situations. Below at a going away party I followed the guest of honor, who very predictably stopped and looked at the people he was conversing with long enough for me to wheel my off camera flash around to the side 45/45 from his nose, then walk back around to grab this shot...
With dual flash the tone of the shadows can be used to control the mood of the lighting from "dark and hard" to "light and soft". Canon ETTL wireless mades it very easy to control. When first getting my pair of 580ex I did this ratio test where I illuminated both sides of a white binder with my Master Fill and the right side with the Slave Key light:
The Canon ratio numbers represent relative incident strength at the subject, not the net result of the overlap as with the portrait ratio convention. So if when a 1:1 is dialed in what is happening is this:
1:1 Master A
1:0 Slave B
Both lights are adjusted by the metering to reflect the same amount of light in the same areas they overlap and the overlap puts 2x more light there than in the areas only the Master fill hits. An A:B ratio of 1:2 produces a net effect similar to a 3:1 conventional H:S ratio
1:1 Master A
2:0 Slave B
But you will only have control over the ratio if you have control over the direction of your key and fill sources.
One of the reasons the lighting is flat looking in your first attempts is your use of the Stofen cap on your off camera "key" light. The cap bounced the light 360° off the walls and ceiling so you really didn't have very precise control over the modeling your key light was creating.
What you will find with experimentation is that when you keep your fill source centered to control the tone of the shadows and overall perception of "softness" you can use more directional key light sources and still get flattering results. Years ago when I learned to shoot weddings with dual flash from Monte Zucker we used direct flash and got flattering results because of the neutral fill.
If you start with very light shadows opened with the centered fill and a A:B = 1:1 ratio you only need to add a small amount of light from the side to create highlights over the those shadows and the net result will look soft if the skin isn't creating any specular reflections from the flashes.
For your next experiments get rid of the Stofen cap on your key light. First try with direct flash as your key light 45/45 from the nose. Then try with a small but directional modifier like one of my DIY designs you can make out of those white cardboard boxes your presents came in and a stapler using the template here: http://photo.nova.org/DIY 01/ Then try a larger modifier on the key light such as an umbrella, but keep it directional so its not spilling light all over.
If you want to see what the modifier is doing and what spill fill is adding to the mix try the same portrait lighting indoors, then go outside at night where there is no ambient light and nothing to bounce spill. By comparing the indoor and outdoor results you'll understand how better how the use of the StoFen cap is affected by all the light it is bouncing around the space.