Getting The Best Performance From Canon's E-TTL Flash System

Canon has a line of flash units to go with the cameras it sells. The two most commonly encountered flash models are the 550EX and 420EX. The 550EX is more expensive, more powerful, and has more features than the 420EX. Both flash models use what Canon calls E-TTL (evaluative through-the-lens) flash control with all of Canon's newer cameras. In a nutshell, when the shutter button on the camera is pressed, the camera signals the flash to fire. The camera measures the effect of the flash and calculates the necessary adjustment to the flash power to obtain the correct flash exposure. The camera then opens the shutter, beginning the exposure, and signals the flash to fire at the previously calculated power level. The flash fires, the shutter closes, and exposure is complete.

The default way the camera reads the metering flash is a type of spot metering. When autofocus is activated, E-TTL measures the metering flash in the areas of the frame corresponding with the autofocus sensors that are currently active--the ones covering what the camera thinks is the main subject of the photo. This prevents the flash metering system from overexposing in many situations, like when  a person is standing close to the camera with a large open area in the background. Since the flash is being metered on the person instead of the open space in the background, the flash power is kept at a sensible level. But there are some situations where this behavior causes inconsistent and undesirable results. At most weddings, the bride wears a white dress, and the groom wears a dark-colored tuxedo. So if the active focus point happens to fall on the brides dress, a low flash power setting will be chosen because the white dress reflects a high percentage of the light striking it, meaning less flash power is needed to properly expose that part of the picture. But if the active focus point falls on the tuxedo, a higher flash power setting will be chosen, because more light is needed with dark-colored surfaces to get exposure within the range of usability. But this means that a series of shots taken of the bride and groom together can have significant variations in exposure level, depending on the exact part of the frame used to measure the metering flash. This drives wedding photographers crazy, because there is a fairly narrow exposure range that will properly record the details of the bride's white dress and the groom's dark tuxedo. Don't despair, though, there is a solution for situations like this.

The first step is to configure the autofocus as specified in 1Ds Autofocus Tips. The benefit of this is simple. When manually focusing, or when the autofocus system is not active, E-TTL measures the metering flash using all of the metering sensors, not just the one(s) associated with the currently active focus sensor(s). So in the bride & groom shot mentioned earlier, E-TTL will measure the metering flash on the bride's dress and the groom's tuxedo simultaneously, thereby increasing the consistency of the flash output. Choosing whether to use all metering zones for E-TTL or just the ones associated with active focus sensors is easy. If you want to use all metering zones to determine E-TTL flash output, press either the * or X button to autofocus if desired, but release the autofocus button before touching the shutter release. That will deactivate the autofocus system before E-TTL starts the flash metering process, and force it to use all metering sensors instead of a select few. If standard E-TTL behavior is desired, press an autofocus button, and keep it pressed while pressing the shutter release. Autofocus will be active at the time of shutter release, and E-TTL will meter where the active focus sensors are located.

Another trick is to use AI Servo focus. In AI Servo focus mode, E-TTL seems to always meter using most or all of the metering zones, regardless of whether autofocus is active at the moment of shutter release. This isn't covered in the manual, so this conclusion is based on my personal experience. I plan on testing this idea further, and will post the results here.

There is another solution, found in the manual, but it is slower than the method described above. The Flash Exposure Lock (FEL) button, when pressed, will fire the metering flash, and the camera will measure it with only the center metering sensor. This method can be very precise and consistent, but is a little awkward because you have to point the camera at something consistently colored at the same distance as the primary subject, (such as a spot on the bride's dress) press the FEL button, re-orient the camera to get the desired composition, focus, and shoot. Re-orienting the camera twice for every shot is makes shooting quickly very difficult, and can cause important shots to be missed. I don't use FEL very often.

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