A shoot-through umbrella works very simply by taking the light from a light source and expanding it’s spread.
A shoot-through umbrella is typically white and a fairly thin or translucent white at that. The light is intended to travel through the umbrella to reach it’s subject.
To show you exactly I’ve put together some diagrams that show where the light goes. For this example we’ll assume that an umbrella is a cut of a perfect sphere. The focal point, through which all the reflected light passes, is 1/2 * radius of the circle, or half way between the sphere’s center point and the edge of the umbrella.
When the light hits the reflective surface it bounces both through the umbrella and some spill (usually about 1f stop) comes back in a straight line through the focal point. The light that gets through the umbrella’s surface travels in the opposite direction of the light that reflects back from the umbrella’s surface.
You can see where the reflected light hits the subject.
At first you’d expect to want to place your subject at the tip of the umbrella (like shown in the diagram) but that’s rarely the best idea – we’ll cover that in another article.
Check out our sister site, Single Strobe Photography to find some great shoot-through umbrellas to buy through some of out favorite online retailers, B&H and Amvona.
I actually consider this description to be wrong and misleading. Although the optical principles described are correct for an transparent material such as glass, it’s not true for an opaque material such as white fabric used in shoot through umbrellas. Instead, every point on the surface of the umbrella turned away from the light source will radiate light in all possible directions (hence, as a hemisphere originating from that point).
What really causes the softening effect of the light and shadows is not that the umbrella is expanding the spread, but that the umbrella is expanding the size of the light source as seen from the object.
A naked flash head, as seen from the object, occupies a very small space in the field of view. Hence, “light rays” originating from the flash heads will come in a very narrow “cone”, creating crisp shadows. From the flash heads pow, the light rays spread out like a cone, but from the objects point of view, the cone of light seems very narrow, since only the rays that “hit” the object straight on is “seen”.
If we throw in a shoot through umbrella between the flash head and the object, what do we get? Well, the “cone of light” from the flash head hits the much larger opaque umbrella. I.e. it makes the back side of the umbrella radiate light in basicly the shape of a hemispere. But, from the objects point of view, the light rays are now originating from a much larger source. The rays are comming in from different angles, therefore creating softer shadows.
Best regards, Christoffer Wallstenius