Matt Laur , Dec 03, 2010; 09:33 a.m.
While an umbrella is indeed an adventure when there's a breeze (let alone wind) outside, this is also true of a softbox. Either way, you've got to have a sturdy stand and sandbags (or, better yet, an assistant!).
A 24x24" softbox isn't going to do much for you outdoors except to eat up the power of the strobe. By the time you move a 2-foot softbox back far enough to fill shadows in a daylight portrait (I assume we're not talking about just head and shoulders shots here, right?), the size of the light source isn't big enough to meaningfully impact the apparent quality of the light ... but you are going to impact the effective power of the flash unit, and you're going to make it a bit harder to trigger the slave using the D300's pop-up as a controller. That's hard enough if you're at any distance in daylight, and harder still if the flash is buried behind something. You can rotate the flash's lower body so that the IR sensor is more favorably facing the camera, but that still doesn't always solve the problem.
If you're looking for a large light source, an umbrella may actually be an easier solution in the field, but they are fragile, no question. Luckily they're cheap! That Lastolite softbox is very well made (I own one, and we were talking about it once on this thread, where I posted a bunch of explanatory images, if you're curious), but spendy.
If you're going to be shooting outside and need your flash as a key light (rather than as fill), then the larger the source the better. Similarly, the closer the source, the better, if it's not as large. In the case of a 2-foot softbox used as key portrait lighting, you might find yourself needing to place it only a couple of feet away from your subject's face. At that distance, the light fall-off over distance means that a two-person shot would have one of the two people at least a stop brighter than the other if the box is off-axis from the camera. It's tricky, ain't it?
Just because I have it handy, below are two out-takes from a shot involving a couple walking down a path in the woods. On the left, you'll see the assistant, with an SB-800 shooting through an umbrella, with the whole rig on a monopod being use as a boom. You can tell that when the couple turns around to walk towards the camera, they're going to be in shadows, and backlit by the sunny hill in the background. Because the umbrella is going to be a fair distance from them, the light from it hits them both more or less evenly. Here, it's just providing some fill, to help deal with the shadows in the woods. The base of the SB-800 is rotated so that the camera's pop-up can act as a trigger, which it did just fine at this distance (around 30 feet).
A walk in the woods.