Moving a light source affects it’s percieved size (hard/soft shadows) and light falloff (dramatic/flat lighting). How do we manipulate these together?
Falloff: Light is more dramatic when it is closer.
Perceived size. Light sources get bigger when they get closer. But when they get closer, the light falloff is more dramatic. This leads to a certain ‘look’ when you move a softbox or an umbrella really close to a subject, soft, smooth, but dramatic light. I happen to really like this light for portraits, as seen below.
Notice how the light falls to shadow smoothly - it’s a gradient, but it falls to very dark shadows very quickly. As you can see in the reflection in the subjects eyes, the light source is very close. Camera right and up a little. It’s so close, just out of frame, that there is a noticable difference in brightness on his forhead and his cheek, just inches further from the light.
What if we move the light source far away, but keep the light source large?
We get something like this. Soft shadows, flat light. You can see in the eyes that the key light is in the same relative place as with the above photo - same direction of light, but it’s much further away. It’s also large enough that the distance doesn’t change the relative size. The light falls to shadow much slower.
In the first photo, the background was pure black. Nothing. Here, the wall is lit by the light source, and is an even grey. If I wanted it darker I could move the wall further away. But, because moving walls is pretty difficult, I would accomplish that by moving the subject closer to the light source, relatively. Either by moving the subject closer to the light source literally, or moving the light source closer to the subject. That would move it literally closer to the wall, but remember, the distances are relative.
One more photo to analyze:
Focus on the key light, the white light, not the red fill light. the light is hard, so it goes into shadow very quickly. You can see the hard shadow line her nose makes on her face. But the light source if far away. She is the same brightness from her head to her shoulder, everything in the light’s spread. The flash if far enough away that there isn’t a change dramatic change in brightness. Contrast that with the first photo above and the bright spot on the subjects forehead.
When I am lighting a portrait, I usually start with my light source direction and distance, and then pick what modifier I want to adjust it’s size last. The reason I often work this way is because I usually don’t shoot in a studio, and I am limited by my environment. I start with looking at how much room I have, where I could put lights, what types of options are available to me. - how far away can I put lights? What color are the walls/can I bounce light off of them? And so on. From there I can usually get to a starting point, and after that it’s just experimenting and iterating.
While the properties of light are intuitive - we have been looking at things that have been light since… ever, they are also complicated. They build on each other and quickly become highly complicated. One’s first task it to learn how to identify what light sources created what image, just by looking at the image. Once you can think in this direction, from image to light sources; it’s easier to learn to go the other way, and pre-visualize what something while look like under certain lighting conditions. Once one can do that, then it’s just practice that brings one to understanding what changes they should make to work torwards a certain look.
This is true for headshots, product photography, and even naturally lit photographs. Even when you aren’t controlling light sources, you have the ability to move the subject and the camera. Or even just the camera. Start looking for light in the world and take advantage of it.