Get the opportunity to take a picture and missing the right light? A nice big softbox would be perfect, but that's back at the studio. Maybe you've got a speedlight, but you stupidly left your diffuser at home? Well where there is a will there is a way. Though there are entire sections of the B&H catalog with all kinds of expensive ways of manipulating your light, you don't need to spend a lot of money or any money at all really. And in a pinch, your imagination can do good things. In it's most basic, light is either diffused or it's not. And that has everything to do with the size of the source of light. Front of speedlight is small and therefore very directional. Sky is big, therefore diffuse. And where there are a ton of ways to make your light bigger, from umbrellas and shoot through umbrellas and softboxes and octabanks and beauty dishes and parabolic reclectors, among many others; For the most part, they're usually pretty subtle variations on the 'make the light bigger' motif.
So there I was at my friend's Hardin and Jenn's this weekend and wanted to take a couple of portraits of them, but didn't have a diffuser with me. Could have used a nice window of course but instead I tried a technique I've read about on the intertubes. So I grabbed a big white plastic bag from under the sink and cut it into a nice 2x4' white plastic sheet. Had one person hold the plastic and I or another person held the flash pointed through is and voila, basically I had a softbox. The light hitting his left shoulder and side of his face was from a window behind him. So, two soft lights for the price of one speedlite.
Is it as soft as a big box with an extra diffusion panel? No not quite. But for free and in a pinch, it'll do. If you zoom into his eye at 100% you even see what looks like a softbox to his right. This is the same lighting that I used on yesterday's 365.2011 of Jenn. In that shot I had the panel and light above the camera looking down as opposed to the side, but that just shows the versatility. Sites like Strobist and the like post this kind of stuff all the time. And while I wouldn't rely on a plastic bag as my main diffusion source on a big ad shoot, it's handy to know it's an option.