The following series of images of Tamra and John were taken with a Canon 7D with a 24-105 f4 L IS lens. I used a set of Pocket Wizards to trigger an off-camera Nikon SB-26 hotshoe flash set at 1/2 power. The SB-26 was mounted on a lightstand to camera right about 10 feet away from the couple, and modified with a Lastolite Ezybox Hotshoe Softbox. A 1/4 CTO gel was added to the flash to warm the light up a little.
Everything on the camera was set manually - the iso was set to 400, and the aperture remained at f4. The only thing that changed between shots was the shutter speed. I started at a shutter speed of 1/30th and gradually increased the shutter speed until the sky darkened to my liking. The total time it to make all these images was less than a minute.
Notice how the exposure level of the couple does not change throughout. This is because the aperture, iso, distance of the light to the subject, and the power level of the flash do not change. Only the shutter speed changes, which changes the exposure level of the sky.
The only limit to how dark I can make the sky is the flash sync speed of the camera. In this case, the 7D has an upper limit of 1/250, which is pretty good (my 5D only goes to 1/160). If I go higher than the sync speed, part of the image will be dark, because the shutter is moving too fast for the flash to expose every part of the subject/foreground. If I were to use a Canon flash, like the 580EX, I could go much higher using the high-speed sync mode.
Some people have asked why I use Nikon SB-26 strobes when I'm shooting with Canon gear. Although I do have a couple of 580EX flashes that I use, I like using the Nikon flashes for a lot of manual flash work because:
a) They have PC sync ports which allow me to trigger them with Pocket Wizards. Only Canon's newer flash has a PC sync port - the 580EXII.
b) The SB-26s have built in optical slaves so if I run out of Pocket Wizards, I can still trigger them with the pop of another flash.
c) They're durable and cheap enough that I don't have a cow if one breaks. I've been able to find them in secondhand stores in Japan for around $100 each. The Canon 580 EXIIs are about 4-5x the cost.