Before you continue, please read
Chapter 1: Facts that every flash shooter must understand.
Chapter 2: (WHY) SHOULD I GET A FLASH UNIT FOR MY CAMERA?
CHAPTER 3: A SYSTEMATIC APPROACH TO BOUNCED FLASH
While flash photography is complex enough that no single strategy works in every situation, this approach should work well in relatively small rooms with low, white ceilings, such as residential, classroom and office settings.
Why use flash?
Simply put, adding light to indoor settings will allow you to use a faster shutter speed (less motion blur), a smaller aperture (more depth-of-field), and a lower ISO setting (less digital noise) than you could use with ambient light only. The focus-assist light on your flash unit will also help with focusing when needed.
Why bounce the flash?
We are accustomed to overhead lighting, so the shadows produced by light bouncing down from the ceiling will seem more natural looking. When the light from the flash hits the ceiling, it reflects down in all directions, illuminating the entire room. This creates a larger effective light source and produces more even lighting, softer shadows, and brighter backgrounds. When properly used, bounced flash will help to create images that don’t look “flashed” at all. Finally, bouncing will eliminate the redeye problems associated with direct flash.
Color temperature issues
Flash units produce a color temperature that resembles daylight. Incandescent (tungsten) lights have a much lower color temperature, and fluorescent lights have a higher color temperature along with other issues. Since most flash units have enough power to completely illuminate small rooms, my general recommendation is to reduce the amount of ambient light as much as possible by setting the shutter at flash sync speed (1/200 or 1/250 on today’s Canon DSLRs). This will make your flash the only significant light source, eliminating the problems caused by having multiple light sources at varying color temperatures.
You want an aperture setting that gives you sufficient depth-of-field, but don’t go overboard here. Smaller apertures (higher f/ numbers) will require a higher ISO setting to get sufficient illumination. DOF is a fairly complex concept, but generally speaking for indoor shots of people, f/4 should be enough for a single subject, and f/8 should work for most small groups. These are very general guidelines and the “best” aperture setting depends largely on your artistic goals.
Now that we have the shutter speed and aperture determined, ISO is the last part of the equation to figure out. Since higher ISO settings tend to produce more digital noise, the trick is to set it high enough to get sufficient light without going higher than necessary. This sometimes requires a bit of trial-and-error, but ISO 400 is usually a good starting point.
Test, chimp, and adjust
It’s often difficult to predict what aperture and ISO settings will be required to get proper illumination with bounced flash, so testing is always a good idea. With your camera in “M” mode and set according to the instructions above, and the flash unit in E-TTL mode, take a few shots of someone on the other end of the room. Immediately after each shot, look for the flash exposure confirmation lamp (FCL), near the pilot light on the back of the flash unit (on Sigma units, the “ETTL” indicator on the LCD will blink for 5 seconds). This indicates that the flash had enough power to create what it “thinks” is a correct exposure.
If the FCL doesn’t light, it means it didn’t have enough power for proper exposure with the settings you chose. You need either a wider aperture (lower f/ number) or a higher ISO setting. Adjust accordingly and test again.
If the FCL lights, take a look at your histogram to determine if the shot is properly exposed. If the image is too dark, dial in some +FEC (flash exposure compensation). The need for +FEC is normal with bounced flash and E-TTL flash metering. After adjusting the FEC, test, chimp, and adjust again as needed. More info on how to read a histogram here.
Once you have made these adjustments, you should be ready to make properly exposed images with bounced flash. But remember to check your FCL and histogram often! Many factors, including white clothing, windows, and changing backgrounds can “fool” the flash metering and require adjustments as you go.
Below is my usual configuration for bounced flash. Here are a few other points to remember.
1) Point the flash straight at whatever you want to bounce off. That means straight up for ceiling bounce. Avoid the 45 degree angle technique. This will tend to light only part of your subject directly and create the "hot spot" on the ceiling directly above your subject. Light from directly above is rarely flattering.
2) A 3 x 5 index card attached to your flash head as shown will create catchlights in eyes and provide a bit of direct illumination to fill in shadows.
3) Zooming the flash head to its widest setting will illuminate a larger area of the ceiling (creating a larger effective light source) and throw more light on the card.
4) Ceilings aren't the only surfaces you can bounce off of. A light colored wall beside or behind you can work too!
Chapter 4 - Guide Numbers and High Speed Sync