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While very well executed in terms of lighting and exposure it has a very "static" look, like a ID photo or mug shot due to her expression, the full-face angle, tight crop, the camera height relative to her face and the dark unfilled shadows which are typical of single flash shots.
There's a dominio effect of variables. What you do with one affects decisions about the others.
Your choice of centered "butterfly" lighting for the full-face pose was a good one. When you aim the nose directly at the camera as you did here, then center the key light on the nose as you also did, it creates a very flattering symmetrical combination. The only problem here is that your light is too low, based on the modeling and the catchlights in the eyes, which are a "tell-tale" for where the key light was placed. Next time increase the height of the light do it is at a 45° downward angle with the catchlight just kissing the top if the black pupil at 12-o'clock and you'll see better modeling of cheekbones, nose and chin — it will not look as flat as it does here.
Moving the key light up will move the shadows downward more — the domino effect.
There's a school of thought that says one light is better to learn lighting. That's true with regards to learning key light patterns, but the problem with working with only one light is you have no independent control of the fill in the shadows. In lighting ratio exposure is always set based on the highlight tone in the skin, which stays constant. What changes from a 2:1, 3:1, 4:1, etc. ratio is the shadow tone relative to that constant highlight tone. The psychological effect of taking a pattern like this starting with no fill, then gradually adding more and more fill from near the camera at subject chin level is that the progressively lighter shadows make the shape of the object seem less angular and "hard edged" due to way the brain interprets the shadow clues.
In addition to changing the illusion of 3D shape the lighting ratio / shadow tone sends clues to the viewer, by inference, about that mood of the subject and the environment they are in compared to how people are seen normally most of the time indoors and out at mid-day. When you see a photo and your reaction is that its not "right" in some way that's your brain comparing what your eyes are seeing to your mental baseline of what a face or object should look like within the context of the background . So while key light position is important for modeling the 3D shape of the face, precise independent control of fill is how you control and nuance the emotional reaction the viewer will have to the lighting pattern the key light creates. The more flat, even, neutral fill you add the "softer-edged" the lighting will look.
The shots above where taken with a 22" dish and 24x32 SB with a 20" circle mask place back near the camera in a butterfly pattern.
What makes the lighting on the face seem soft is the fact the key and fill are at the same incident strength. The key light overlaps the fill in the highlights producing a very shallow 2:1gradient of reflected brightness on the face:
1:1 Even fill from chin level
1:0 Same incident strength key light hitting at downward 45° angle
2:1 "soft" looking 2:1 lighting ratio
To arrive at the best choice for the lighting ratio start with thinking about the age and gender of your subject and the mood you want the expression to project.
Kids will look younger and childish in low ratio lighting like 2:1 where key and fill are equal strength. As you increase the lighting ratio it will cause the kids to seem more grown up and mature. For example for a set of confirmation photos conveying a boy becoming a man I started with 2:1 butterfly lighting for this full length view...
Darkened the shadows a bit for this one...
and finished with this darker, moodier higher ratio with very little shadow detail...
Women of all ages will be flattered by 2:1 ratio lighting where key and fill and equal and overlapping. To make them seem more business-like or "tough" and able to "play with the boys" you'd make the shadows darker, more similar to how you'd light a man or older boy.
What winds up looking more or less "normal" in terms of lighting ratio is a 3:1 numerical ratio which results when key light is 2x brighter in incident strength than the fill:
1:1 Even fill centered at chin level
2:0 Key light 2x (1-f/stop) brighter than fill
3:1 Produces an average / normal baseline ratio...
This is typical of how a portrait with a 3:1 ratio looks....
Pretty normal, no? About how you'd see him if you met him in your office at work, or on the sidewalk outside. That what I mean about 3:1 being the "baseline normal" in terms of emotional reaction to lighting ratios. When you make the shadows darker that 3:1 you shift the mood / context from "normal" to more serious / thoughtful / scary / dangerous / sinister, as if lurking / emerging from the shadows. It's an emotional reaction based on the experience of scary things happening at night.
Numerical ratios are like blueprints. If you've never built a house before you'll want one. If don't know what a 2:1 vs a 3:1 ratio looks like you'll want to at some point set one with meter or by power and distance so you'll know. But what is more important is looking at the results of your key and fill placement and relative power and asking whether the results are age/gender/mood appropriate for the impression you what the viewer to have of the person in the portait — tough as nails, or soft as a pussy-cat? It's mostly as matter of fill power and placement to control the shadow tone. That's much easier to control with separate key and fill flashes.
The pose looks "static" here because the eyes and mouth are dead level with the frame. As with light ratios there is a right or wrong way to pose just cause and effect as to how the pose will be interpreted. When the planes of the body are level — eyes, shoulders, hips — which occurs when people stand flat footed and square to the camera the body language created in the photo is a tense "I'm not moving" static and defensive. When you angle the eyes, shoulders, and hips the body language is more relaxed. For example compare the two shots of the girl in the riding outfit. In one the eye line is tilted in the other it is level. But in both the shoulder line is angled.
Camera height relative to face:
One of the more distracting and least flattering features on a face are the nostrils. They are distracting because they are dark and contrast strongly with the highlights on the nose, cheeks and upper lip surrounding them.
The way you can control the rendering of the nose is by changing the height of the camera relative to the face. If you can see the nostrils in the viewfinder it should serve as a reminder to check the view from higher up. I also have a small step stool handy when I shoot portraits so I can get the camera above the tip of the nose looking down.
Noses come in all shapes and sizes. Something I've noticed about actors is most tend to have the type of nose shape where the tip is below the base of nose which naturally hides the nose holes when the camera is level. The more the nose is upturned the more you will need to get the camera above it to try to hide the nose holes. For some noses you can't completely hide them but less of them you see the more flattering the portrait will be.
It's also a good practice to shoot from above your subject with them looking up because it stretches the neck, slimming it, and makes it easier to get the fill light under it to control the tone of the shadow there. A shadow under the chin is natural, because natural light comes from overhead, and the contrast of the shadow on the neck is what defines the 3D shape of the chin in the 2D photo, but you don't want it to become a noticeable distraction.
When you stand on a stool looking down with the subject looking up at the same angle, the camera / face angle winds up the same as if standing on the ground facing them // vs. | | - parallel planes, so there is no distortion of the face, but it will be better lit and be supported by a neck that looks slimmer and longer.
The problem with tight head shots like this is there is no visible means of support. It becomes a "floating head". The reason it winds up looking like a drivers license or passport photo is that's how they are typically taken. But even these guidelines for passport photos from the State Department show shoulders supporting the body..
Again there's no "rule" governing how much body you show, but you need to give some thought to how the viewer will react. You know the subject and what she looks like full length in all her glory, but all you are showing us strangers her is her chopped off head on a platter. Is that the "first impression" you want her to make?
The take away here is to start with the goal of the image and mood you want the subject to project in the photo. Happy / sad, willing to engage / wanting to be left alone, demure / sexy, playful and relaxed / serious and tense, etc.
Once you have that goal clearly in mind then it becomes your blueprint for picking the body angles / pose and lighting ratio that will convey that message in the photo and match the expression. Portraits "click" when the expression, tone of the shadows and body language of the pose match each other and the background context of the environment the person is seen in for environmental / candid shots. It's not about rules, just consciously understanding the sub-conscious clues you've been reacting to your entire life