II. “wrap:” source size and distance.
If there’s one universal dictum in achieving soft lighting, it’s that “bigger is better.” Over-sized softboxes, umbrellas, scrims, and reflectors are all designed to do one thing: To produce the maximum amount of wrap.
The larger the source, the greater the wrap. The greater the wrap, the “softer” the light.
Recall the near-shadowless image of the foam forms? It’s the light falling “around” the geometric shapes which is the result of “wrap,” and it’s this light which fills in the shadows around the objects which makes the light appear “soft.” The concept of wrap describes the light which surrounds and envelops your subject from many different angles.
The image above was lit using a 54″ x 72″ softbox, oriented horizontally, set only about five feet from the subject. Because the softbox was oriented horizontally, the large width of the source allowed its light to wrap around the “sides” of the mostly vertically-oriented subject.
The closer the source to your subject, the larger its size, relative to the subject, and therefore, the “softer” the light.
source size is relative:
Note that source size is relative. The closer the source is to your subject, the larger it appears to your subject. The further away the source is from your subject, the smaller it appears. Again, the most common example is the sun. A body more than a million times the size of Earth, appears as only a small spot in the sky. Due to its considerable distance from Earth, not only are its rays virtually parallel, its relative source size is very small. At that immense distance, the sun more resembles a “point-source” (a point-source is any light source which emanates from a very “small” source, relatively speaking; e.g., a bare light bulb, your on-camera flash, the sun, etc.).
As the source is moved closer to your subject, the source becomes “larger.” All else being equal, the larger the source, the “softer” the light.
When you move your source closer to your subject, its relative size to your subject becomes larger. The larger the source, the greater its ability to scatter its light rays around your subject, filling in all the nooks and crannies. This “filling-in” reduces shadow area, and creates a more even distribution of light over your subject, lowering overall contrast. This lowered-contrast is what produces soft light–where the differences between the highlight and shadow areas are diminished.
Again, as you can see from the illustration at right, the source’s rays are able to reach all around the subject, filling in as much shadow area as possible. This happens when the source is large enough for its light to surround the subject, and is why larger sources appear softer.
Now that you have a better understanding of what makes light appear “soft,” let’s move on to the third, final, and most interesting concept: light “direction:”