Using an External Flash

The 550EX flash.

The small automatic flash built into your camera is convenient, however its range is short and it is so close to the lens that photos of people often capture them with red eyes. It also emits a hard, direct light and can't be rotated to bounce flash off a wall or ceiling to soften it.

For better flash photography you need a hot-shoe mounted Canon EX-series Speedlite such as the 550EX, 420EX, 220EX, MR-14EX, MT-24EX, or ST-E2. Any of these can be mounted on the cameras hot shoe or attached by a sync cord for off-camera use. When using these flash units, the camera controls the exposure just as it does with the built-in flash. (You can't use both the built-in and external flash at the same time so close the built-in flash when you attach the external one.) One of the biggest advantages of these units is that they let you swivel or rotate the flash head so you can bounce light off walls and ceilings. This lets you get softer light on the subject so contrast is reduced and hard shadows are minimized. Let's take a look at some of the features you'll have access to on EX flash units. All of these features are available with the 550EX, but not all are available on other units.


■ The built-in flash doesn't support FP high-speed sync operation flash. To use this feature you need a compatible EX flash such as the 420EX or 550EX.

■ When using an external Speedlight, it may emit an AF-assist beam so the camera can focus in the dark. The camera may not be able to focus if you have selected an off-center focus point (page 55).


The shutter speed you use when shooting with flash is important. When you take a flash photo, the first shutter curtain opens to begin the exposure, then the second curtain closes to end it. At shutter speeds above 1/200 the second curtain starts to close before the first curtain is fully open. As a result, a "slit" formed by the two curtains moves across the image sensor and normally only a part of the image can be captured by the brief burst of flash. The rest of the sensor is blocked by one or both curtains.

To get a fully exposed image at fast shutter speeds, the flash must fire when the shutter is fully open. This timing between the flash and the shutter is called flash synchronization or X sync. On the Digital Rebel, the shutter is fully open only at shutter speeds of 1/200 second and slower. Faster shutter speeds require what's known as high-speed sync flash (also called FP or focal plane sync). High-speed sync can capture a fully exposed image because the flash fires repeatedly as the "slit" moves across the image sensor during the exposure. The only drawback is that the flash power is reduced so you can't be positioned as far from a subject. The higher the shutter speed you use, the closer you have to be.

Since this feature lets you use flash at shutters speeds as fast as 1/4000 second, Canon lists three situations where you might find it useful:

■ When using fill flash out of doors, you can use a fast shutter speed to freeze action, or a wide aperture to throw the foreground or background out of focus.

■ When doing a portrait and want catchlights in the subject's eyes.

■ When using fill flash to lighten shadows.

The flash exposure bracketing icon.

The flash exposure bracketing icon.

The 550EX Speedlite was developed specifically for Canon's EOS cameras and takes full advantage of those camera's autofocus (AF) technology and E-TTL autoflash. The flash has a maximum Guide Number of 180 (ISO 100, ft.), an AF-assist beam which links to the Digital Rebel's 35-point area AF, FP Flash (high speed sync), FE lock (a flash version of AE lock), and FEB (Flash Exposure Bracketing).

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