A Flash Primer
Apparently my instructor believes the detachable electronic flash is one of the most useful accessories a photographer can have and should almost always be used. Each manufacture makes flashes with slightly different settings and features....so the below is an overview.
The Three Basic Exposure Modes
Manual - The output of your flash is expressed as a guide number (GN). The guide number is calculated by multiplying the f-stop required to make the correct exposure at 10 feet with ISO 100 film. If at 10 feet your flash makes a correct picture when the lens is set at f8, then the guide number is 80. Once you know the guide number, you can calculate an exposure for any distance with the simple formula: GN / distance = f-stop. Or even easier you can use the exposure dial calculator on your flash.
Auto Sensor - Most flash units have a small auto exposure "sensor" on the front of the unit. This sensor measures the light bouncing off your subject and turns the flash off when the corret amount of light has been detected. On most flash units you can tell the sensor what ISO you are using and what f-stop you would like to shoot at and the sensor will do the rest.
Through the Lens (TTL) - Your digital cameras have an amazing system to measure the flash exposure through the lens, while the exposure is happening. Like the auto sensor mode, the light leaves the flash, bounces off the subject and is read by a sensor. As the name implies though, the sensor in the TTL mode is located inside the camera to give you the most accurate exposures possible. Using the TTL mode will allow you to shoot flash pictures automatically, including tricky fill flash situations, with accurate results. The key to using the flash in the TTL mode is in using the over/under adjustment. Just like the camera's meter needs to be adjusted using the exposure adjustment override, the TTL mode needs a little help in achieving the correct exposure.
Fill Flash - When the flash is used to bring up the amount of light in the shadows.
A bright sunny day is very high contrast light. If you want to photograph someone with the sun behind them chances are you will have to choose between overexposing the background in order to expose for the face or having the face in shadow to allow a correct background exposure. Fill flash allows you to keep the natural exposure for the background and edge light and use the flash to match the exposure on the face.
How to modify the light....
Take it off the camera! Why do direct flash pictures sometimes look harsh? Light coming from just above the lens is rarely flattering to our subjects. The answer is to purchase a long "sync" cord and change the direction. My instructor recommends a sync cord that is 3-5 feet long to begin with. That length will allow you to hold the flash in one hand and the camera in the other. If you have a TTL flash and camera make sure you spend the extra money on the dedicated TTL cord.
Bounce it. The bigger the light source, the softer and more diffused the light will be. Many flash units will pivot so you can bounce the light off a ceiling. Some pivot so they will point towards a wall. With automatic modes you can rely on the equipment to figure the exposure. Remember when bouncing the light you loose a substantial amount, so choose a surface that isn't too far away. Also, remember that the color of the surface that you bounce off of will be the color of the resulting light.