Paul Buff
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A white BD with the usual blocker is basically a non-focused round light source that that projects a wide, even beam essentially identical to what a round white bounce umbrella would project. Nothing contrasty about it.
What is perceived as contrast is the fact that when it, or any similar size modifier . . . . umbrella, softbox etc. is placed close to the subject, inverse square law causes the parts of the subject closest to the subject to be brighter than the more distant parts. Thus the subject has gradients of brightness across it . . . a light side with gradients to a dark side. This is not a shadow - it's a light gradient. The further the light is from the subject, the less the inverse square law falloff but the more defined the shadow because the wrap around effect is diminished. The same effect occurs with a softbox or umbrella except the catchlights are not round.
Also, there is the subject of face-evenness of the light source. Irregularities across the face effect specularity - the brightness of reflective portions of the subject (shiny parts) VS non reflective portions. This also causes variations in the shadow structure and of light gradients when used close to the subject.
Still further, there is how the rays of light emanate from the source. Almost all white light sources produce non-coherent light rays that scatter in every direction from each point on of the radiating surface. But metalized modifiers produce rays that coherently radiate in a different direction from each point of the surface. Depending on the modifier design, the rays may cross . . . in front of the subject, at the subject or behind the subject, or a combination. This adds a further element to the specularity, apparent contras, gradients and shadow structure.
A beauty dish without a blocker, or a conventional metal reflector produces a whole new effect . . . a bright, point source in the center and a ring of large-source "soft" light around the perimeter. Thus it is two sources in one and combines the soft look of a large source with the extreme specularity of the bright point source. Where and this is useful or detrimental depends on subject and the lighting goals.
Finally, there are silver umbrellas. The light that actually strikes the subject is a series of eight spokes of relatively specular light (high spot intensity) that graduate into a field of much lower spot intensity, together with lots of cross angles of emission. This yields the shadow wrapping effect of a large source with a fairly strong accentuation of of shiny portions of the subject (specularity), and yields up to a snappier looking shot than an all-white, even spot- intensity source such as a softbox of white umbrella or white beauty dish.
Finally, the PLM produces 16 spokes of lower spot intensity light than the standard eight spokes. This produces a more controlled, yet still very present, specularity, high shadow wrapping and, in most cases, catchlights that appear round rather than octagonal catch lights.
Diffusing the output of a silver BD or umbrella effectively removes most of the characteristics discussed above and yields a look very similar to a white umbrella or a softbox . . . relatively even spot-intensity and non-coherent, wide angle soft illumination. The over use of large, strongly diffused light sources often leads to mushy, boring results in the view of many pros.
So it is very difficult to make generalizations about the behavior of a particular light modifier. Embry is pretty much on target, but I would add you also have to understand the myriad characteristic differences in modifier types and use them to your advantage. But another method, successfully used by many artists, is "screw the rules and just experiment."
Broken rules often produce the most unique and interesting results. Nine poor shots and one sensational one beats ten mediocre shots in my book. Live light to the fullest
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