This is a Nikon SB800 flash unit attached to my OPUS portable light stand, clamp and 3 foot white umbrella. On the left, the flash head is set to its widest 24mm setting and the umbrella is about 1.5 feet from the clamp, which is near the maximum distance its centre rod allows. The edges are not illuminated. Putting the built-in flat diffuser panel of the SB800 over the head fills the umbrella to the edges uniformly, but you can see it's a bit dimmer too. I've since found I can fill the umbrella if the flash head is set to 24mm without the diffuser panel but the umbrella is placed as far from the flash as its centre-rod allows.