Canon XTi Flash

Canon Rebel XTi Flash

The Rebel XTi's built-in flash has a guide number rating of 43 feet (13 meters) at ISO 100, translating to a range of about 15 feet at ISO 100 with an f/2.8 lens. (Reasonably powerful, but not dramatically so.) The Canon Rebel XTi gives you a great deal of control over flash exposure, allowing you to adjust flash and ambient exposure independently of each other, in one-half or one-third EV increments. This makes it very easy to balance flash and ambient lighting for more natural-looking pictures. The Rebel XTi also uses E-TTL II control for both the built-in and compatible external flashes (according to Canon this includes the older 550EX flash, as well as the newer 580EX; though the 580EX offers even more), a new standard that promises, and seems to deliver better, more balanced exposures. Custom Function 08 turns this mode off and returns to an average metering system. E-TTL II is only available with the built-in flash or when the camera is paired with either the 550EX or the new 580EX flash.

Another nice touch is the Flash Exposure Lock button, which fires the flash under manual control before the actual exposure, to determine the proper exposure setting. This struck me as very handy, akin to the more conventional autoexposure lock function for handling difficult ambient lighting conditions. A Flash Exposure Compensation feature controls the flash exposure +/- 2 stops in 1/2 or 1/3-stop increments.

Several of the more impressive features of the Canon flash system depend on the dedicated 550EX or 580EX speedlight. Among these are true FP (focal plane) flash sync, flash exposure bracketing with external flash units, flash modeling, and E-TTL II exposure control. FP sync requires a flash unit to provide uniform light output for a relatively long period of time, long enough for the focal plane shutter curtain to fully traverse the sensor plane. On the Canon Rebel XTi, this requires a flash duration of 1/200-second. Uniform, long-duration flash pulses like this permit use of shutter speeds as high as the 1/4,000-second maximum that the Canon Rebel XTi is capable of. This can be invaluable when you want to exclude ambient light from the exposure. (FP sync mode is referred to as "high speed" mode on the Canon 550 and 580 flash units.)

Here's the rundown on Canon Speedlights and their compatibility with the XTi:

You'll note the references to "E-TTL remote" capabilities in the table above. Canon's Speedlight system permits TTL flash metering with multiple remote units, and even allows you to set differential power ratios between the slaved units, over a six-stop flash exposure range.

The "Flash Modeling" feature of the 550EX and 580EX speedlights is quite useful. With a F550EX or F580EX connected to the XTi, pressing the camera's Depth of Field Preview button causes the speedlight to fire at 70 flashes per second for about one second. This creates the illusion of a constant light source for your eyes, letting you preview the lighting on your subject when the flash fires. VERY handy, and likely to save lots of shoot/check/reshoot time!

As alluded to above, the "X-sync" speed of the Canon Rebel XTi is 1/200-second. (This is the maximum shutter speed that can be used on the Rebel XTi when working with a non-dedicated, FP-capable speedlight.) When used with higher-powered studio strobe systems, Canon recommends a maximum shutter speed of 1/60-second or slower, to accommodate the time/intensity profile of such units.

A final benefit of the dedicated Canon speedlights is that they carry powerful autofocus assist illuminators that can extend the range of the built-in AF assist light of the Rebel XTi. For example, the AF assist beam on the 550EX is rated as good to about 50 feet, versus the roughly 13 feet of the pulse flash on the Canon Rebel XTi itself. (As mentioned above though, note that the ST-E2 wireless sync transmitter can also be used for AF assist during non-flash photography, a handy trick.)

Canon's E-TTL II flash exposure system also works with certain lenses to include object distance data into its calculations so it can adjust the flash power accordingly. A preflash is fired and the resulting readings compared to the ambient light reading for each of the camera's 35 metering zones from just prior to the flash, to identify and compensate for specular objects (that is, very reflective surfaces). In instances where most cameras would underexpose an image because of a reflective object in the frame, the Rebel XTi will ignore the brighter areas and expose the subject correctly in most instances. This is designed to help shooters like event photographers--especially wedding photographers, whose cameras are constantly forced to balance a bright white dress against all manner of reflective materials on the clothing of others, in addition to the usually black tuxedos of the groomsmen.