A studio umbrella is one form of bounce strobe
The light from flashes, or strobes, can be a direct, harsh light that isn't ideal for portraits. This light can result in hard-edged shadows both in photography of people and of products or objects. These problems can be mitigated by using a diffuser. Diffusers cause light to be more random, or less columnar. This more random light causes softer shadow lines and can be good for reducing facial lines in portraiture.
Bounces
A common form of flash technique is to bounce the light from another surface. Some on-camera hot shoe flashes can be angled to reflect from a ceiling or nearby wall. The effect of the light bouncing from a light, preferably white, surface is a diffusion of the light from the flash. If your flash doesn't angle, you can sometimes attach a white card or piece of paper to the flash to make a temporary bounce.
In studio photography, a common bounce diffuser is a studio umbrella. Studio umbrellas have irregular surfaces that reflect the light in more random directions, thus creates a diffusion of the light. Umbrellas can also be covered with diffusion cloth for more diffuse light.
Shoot Through
Shoot through diffusion is any material where the flash is fired directly through the material. There are many different shoot through diffusers for on-camera flashes. Some are built in plastic plates that flip down over the front of the flash. Some flashes support the use of thin plastic inserts to diffuse the light. A number of third-party flash diffusers work using techniques similar to studio soft-boxes. These are fit over the front of an on-camera flash to create a larger, lower powered diffuse light source.
There are many forms of studio shoot through diffusers. One common diffuser is a soft box. Soft boxes range in size from a foot or two square to six feet long and wide. Some soft boxes are round or hexagonal. In some cases, shoot through studio diffusers have multiple layers of diffusion material. If you need a diffuse light, but don't have a diffuser available, you can make one by hanging translucent cloth in front of your studio flash.
Diffusion Control
Because diffusion increases the light spread, it can sometimes cause light to fall on unwanted areas. If you are using a small diffuser on the front of a studio strobe, you can add a snoot, barn doors, or other method of shading areas of where you don't want light. Large diffusers, like soft boxes, can be more difficult to control. For large diffusers, you may have to resort to large panels to keep light off unwanted areas.
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