I use clamshell lighting in my large format portrait work. Typically these are full face portraits with the lights, lens and subject all on the same axis. Most examples of clamshell lighting that I find online back this up. Is clamshell lighting typically only used in this set up and for full face views?
What if I want to turn the head slightly to get, say, a three-quarter view of the face (to the point where one ear disappears). Should the lights move with the subject? I could see this being the case with true butterfly lighting where there is a prominent shadow under the nose (lest the butterfly disappear). But with clamshell lighting where there is good fill from below and virtually no shadows, is this necessary? Or should I introduce a lateral key and fill at that point?
You will probably have to move the lights as you move the subject's face. Not by much, but as the subject tilts and pans her head, the shadows and catch lights change. Each change may be small and the results subtle, but that may be up your alley.
The shadow is ALWAYS opposite of the light source and behind the subject. If the subject turns 90 degrees, the side of their face would be lit and the opposite side dark. However...
Clamshell lighting simply gives even top and bottom light, and often soft wraparound on the face. There isn't much falloff to the sides if the light sources are large and close, so you may not need any additional fill.
If you use something smaller and harder like beauty dishes instead of, say, softboxes, the light will be more directional and be a narrower beam viewed from the top. Falloff will be quicker to the sides and you might need fill when turning the subject.
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Try a couple of "ring-arounds" series of shots. A "ring-around" is a handy way of finding the best angle for rendering a person's body features, lighting, apparel, background, prop positions, etc. The number of photos and the angular differences will vary. I will typically shoot one head-on and several variations in steps for right and left positions to find a subject's "good" side. This is a good thing to do with every new model/costume/style.
Series #1: With fixed lighting and camera positions, turn the subject by small amounts from one ear (hidden) to the other ear (hidden). Take a shot with each incremental turn. Take notes. Examine your results.
Series #2: Do a similar thing with a fixed model position and lighting. Move the camera around the model by small amounts to get the same (roughly) angles as above. Strive to keep the same subject distance to make comparison easier. Take notes. Examine your results.
The best case study I have found is: Christina Aguilera - Pero Me Acuerdo De Tí Music video
If you take a look at it, every question you asked has some kind of answer.
It is using a clamp shell lighting.
only used in this set up ... for full face views
Here are a middle shot and a wider shoot. And they both look nice.
I want to turn the head slightly to get, say, a three-quarter view of the face
This images are amazing... And the light is in the same place with respect of the camera.
On this, we do not have a highlight to reveal the clamp light, but the softness produced still is the main goal.
Should the lights move with the subject?
We know we have a clamp light setup here... but the camera is off-axis to the lights, and now it looks only as lateral light.
As we move the camera away from the lights, the clamp-shell setup becomes irrelevant. It turns into lateral light and almost a backlight.
should I introduce a lateral key and fill at that point?
The good news is that Photography is a creative process. A light setup is a starting point, so do what the image asks you to do.
My conclusion is that "clamp lighting" should stay aligned with the camera, regardless of the position of the subject.
I am not saying "This should be or not be" Just prepare a light setup that looks amazing.
Screen captures under "fair use" usage.