Using flash for low key shots
Personally I love low key shots. I feel it sets the mood and lets the subject BE the subject!
Lets get started!
For this type of shot we need black background or at least something dark. Now the rule is the further away the subject and light is from the background the darker the background gets, this is due to light fall off from the flash. Even a white background can turn very dark if far away enough.
Now we put the model in position and the soft box or umbrella etc. Usually at a 45 degree angle an arms distance is a good starting point. Of course you may add a fill light or white board, reflector etc at the other side of the model. This is personal preference.
Next is the settings! I know you are excited about this part and probably skipped straight to it!
Set the camera to its max sync speed, usually 1/200 or 1/250 and around f8 or more. If you take a shot without flash it should be completely black, this is okay, this is what we want!
Next is the flash settings! Now you can use a light meter or you can chimp away. If using a light meter you should set the ISO (same as camera) and shutter speed only, When you take a shot with the light meter pointing at the light source (flash) it will say f5.6 or f2.8 or f12 etc etc you should change the flash power until you get to f8, the same as what the camera is. That's it you are ready to go!
Chimping is when you just take a shot and see it is too dark or too bright so you adjust, take a shot and repeat. It is sort of cheating and you aren't learning. This is okay in some circumtances
Change the ratios to suit your needs!