1. The broader the light source, the softer the light.
The narrower the source, the harder the light. A broad light source lessens shadows, reduces contrast, suppresses texture.
A narrow light source does the opposite. This is because, with a broad source, light rays hit your subject from more directions, which tends to fill in shadows and give more even illumination to the scene.
2. The closer the light source, the softer the light.
The farther the source, the harder the light. This stands to reason: Move a light closer, and you make it bigger—that is, broader—in relation to your subject. Move it farther away, and you make it relatively smaller, and therefore more narrow.
Think about the sun, which is something like 109 times the diameter of the earth—pretty broad! But, at 93 million miles away, it takes up a very small portion of the sky and hence casts very hard light when falling directly on a subject.
3. Diffusion scatters light, essentially making the light source broader and therefore softer.
When clouds drift in front of the sun, shadows get less distinct. Add fog, and the shadows disappear. Clouds, overcast skies, and fog act as diffusion—something that scatters the light in many directions. On overcast or foggy days, the entire sky, in effect, becomes a single very broad light source—nature’s softbox.
4. Bouncing light acts as diffusion.
Aim a narrow light source at a broad, matte surface—such as a wall, ceiling, or matte refiector—and it not only refiects the light but also diffuses it by scattering it over a wider area.
Use a shiny refiector, though, and the light will stay fairly narrow on the bounce. The most extreme type of shiny refiector—a mirror—will keep the light focused pretty much as narrowly in the refiection.
5. The farther the light source, the more it falls off— gets dimmer on your subject.
The rule says that light falls off as the square of the distance. That sounds complicated, but isn’t really. If you move a light twice as far from your subject, you end up with only one-quarter of the light on the subject.
In other words, light gets dim fast when you move it away— something to keep in mind if you’re moving your lights or your subject to change the quality of the light.
Also remember that bouncing light—even into a shiny reflector that keeps light directional— adds to the distance it travels.