Canon 40D Flash

Canon EOS 40D Flash

The Canon 40D'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 40D 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 Canon 40D also uses E-TTL II control for both the built-in and compatible external flashes. E-TTL II is only available with the built-in flash or when the camera is paired with more recent Canon EX-series strobes.

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 EX-series speedlights. 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, which the flash achieves by pulsing its high speed strobe multiple times as the two shutter curtains travel across the sensor plane, revealing only a small section of the sensor at a time. Uniform, long-duration flash pulses like this permit use of shutter speeds as high as the 1/8,000-second maximum that the Canon 40D 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 Canon 40D:

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, 580EX, and 580EX II speedlights is quite useful. With a 550EX or 580EX connected to the Canon 40D, 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 40D is 1/250-second. 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 Canon 40D. 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 40D. (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 Canon 40D 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.

The Canon 40D can also control some aspects of an external flash from the Flash Control menu. You can set the flash to not fire when mounted and powered on, which is useful for using the flash as an AF assist beam. You can also set Flash Exposure Compensation, and set E-TTL II to Evaluative or Average.