In the past blogs on this theme, we covered the why and equipment. Now, we get into the meat of this blog: actually doing something.
What is important to know at this point is that any flash you use will be harsh. As a photographer, you will want harsh and soft light sources. You have experienced the harsh and soft light with a normal clear day being harsh with its shadows and an overcast day with lots of clouds being really soft with its light. As a photography, I prefer soft light. On the other hand, you can’t always have it like you want it. What should be important from this is that you need to modify your light source, a flash or small point light source, to the type of light you need. This may need to be an umbrella, softbox, or other diffuser to create the softer light. Remember this as you use any flash or point source type device.
You will want to have a good light meter. Without, you end up guessing and taking time with the different settings you need and can take quite a while. With a light meter, it is a simple click of a button and getting a reading. The key to using a flash is getting the right balance for the tone you want with the background light. Remember the exercises in the last blog? This is why it was suggested you spend the time playing with this. Most will only use the flash units as a fill flash. Others will use them as the main and fill flash. And even others will just use the flash units as the main lights. You are the artist and creating the images – you will know best on how to use them. Play with them, and find out what is best for you.
To use the light as a fill flash, you will need to meter background and set the camera to that exposure value. Without the flash, the subject will be in shadow and underexposed. You will then set the flash where you want it to be (where the shadows are appealing – in portraiture it is frequently high up and at a 30-60 degree angle to the camera). Use your meter to check the light from the flash by measuring with the meter pointed to the flash. Your flash should always be in manual mode when you work with your flash off camera. The aperture value should be 2/3 stop to 1 and 2/3 stop lower, depending on the amount of shadow you want.
As a key light, you will do a similar type of metering for the background. I would highly recommend that you underexpose the background by a 1/3 stop. Next, set the flash up and measure the source with right at to 3 stops over based on what you need. You will find you have to adjust the background exposure to adjust the atmosphere you want.
I hope this helps you in understanding more about off camera flash, the equipment, and how to use it.
Edit: If you are using TTL flash metering mode to control your flash units, _ALWAYS_ keep your metering in matrix mode or the equivilant. This will give you the best results for the images you create. But, keep in mind what you are shooting, and know you may have to adjust the exposure slightly to compensate for the lighting you encounter (all black outfit on all white background, etc.) If you read the posts earlier, you should try to move off the TTL mode as soon as you possibly can for more control and flexibility with the images you create.