Umbrellas soften, broaden, and diminish the light output of any tungsten or flash light source. Umbrellas with a white interior will soften and weaken light more than an umbrella with a silver lining. A white umbrella without a black backing may also be used as a makeshift softbox, although the use of the light will not be as efficient as with a softbox.
Choosing umbrella size is determined by the size of the subject, and the strength of the light supply being used. Choosing an umbrella surface is as subjective as choosing a paint and brush.
This size is perfect for a 1-2 person head, or head-to-waist shot, or a 4×6 ft product setup.