I’ve noticed that the marketing department has gotten a hold of the term “parabolic.” Photographic umbrellas are popping up with that name rather generically attached to them, when they aren’t really parabolic at all.
A parabolic umbrella is designed to reflect the light from a flash back in a focused beam. They are always silver lined, and the intensity of light can be up to 16 times the light emitted from a softbox using the same flash. The light itself is a unique blend: punchy, and at lower power will focus in a similar manner to a grid, with a gentle feathering into darkness. For that reason, they tend to be pricey. Parabolic reflectors have to be very precise to do this: a precise number of panels, precise depth in the umbrella, and precise placement of the flash to get the correct bounce.
Well, really there are two types of faux parabolic. One is a watered-down parabolic; the other just false advertising. Umbrellas with a parabolic structure but lined with white interiors are of the watered-down variety. The white interior spreads out and softens the beams; they’re only marginally more focused that plain ole’ umbrella.
Sadly, some manufacturers are falsely labeling white umbrellas as “parabolic” for the sake of advertising. White umbrellas do not focus the light, and therefore don't have parabolic qualities. White umbrellas with black backs do not focus the light or have parabolic qualities, they simply limit the "spill" that results from light passing through the umbrella.
If you compare your average white umbrella to a parabolic, it’s easy to forget they are great tools for shaping and softening light. Don't disparage your plain ole white umbrella. For the greatest flexibility, get one with a removable black cover to better control the light.
What to Buy
There’s no reason not to buy a plain white umbrella. They’re cheap. They’re great. For a destination wedding I actually bought a compact white rain umbrella. It worked great and fit inside my carry-on.
If you’re going to buy a parabolic umbrella, my advice is don’t got half way. You can get a white lined/black exterior non-parabolic umbrella pretty cheap, and they’ll perform much the same. If you buy a parabolic, buy silver-lined and do your research. Paul C. Buff’s PLM line are reportedly true parabolics, and they’re quite cheap. Westcott is also known to be true, but pricey. The tests I’ve seen on the Flashpoint silver umbrella looks promising, too. There’s a quick test to see if it’s a true parabolic. Set up your flash/umbrella and dial it down to its lowest output. Aim the umbrella at a blank wall. Take a picture. The illumination should feather smoothly into darkness. Then take a picture of the inside of the umbrella (using your flash, of course). The umbrella should be lit with distinct ribs-- not all white.