Both the onboard flash and Canon's EX-series Speedlites employ E-TTL II technology. E-TTL stands for Evaluative Through-the-Lens flash exposure control. E-TTL II is a flash technology that receives information from the camera including the focal length of the lens, distance from the subject, exposure settings including aperture, and the camera's built-in evaluative metering system to balance subject exposure with the ambient light.
In more technical terms, with E-TTL II, the camera's meter reads through the lens, but not off the focal plane. After the Shutter button is fully pressed but before the reflex mirror goes up, the flash fires a preflash beam. Information from this preflash is combined with data from the evaluative metering system to analyze and compare ambient light exposure values with the amount of light needed to make a good exposure.
In This Chapter
Exploring flash technology
Using onboard flash
Modifying flash exposure
Using flash control options
Then the camera calculates and stores the flash output needed to illuminate the subject while maintaining a natural-looking balance with the ambient light in the background.
In a step-by-step fashion, here is the general sequence the 40D uses when you make a flash picture.
1. Pressing the Shutter button halfway sets focus on the subject and determines the exposure needed given the amount of ambient light.
2. Pressing the Shutter button completely fires a preflash so that the amount of light reflected off the subject can be measured.
3. Very quickly, the camera compares and evaluates both the ambient and preflash readings and determines the proper subject and background exposure.
4. The reflex mirror flips up, the first shutter curtain opens and the flash fires exposing the image on the sensor, the second shutter curtain closes, and then the reflex mirror goes back down.
In addition, the flash automatically figures in the angle of view for the 40D given its cropped image sensor size. Thus, regardless of the focal length of the lens being used, the built-in and EX-series Speedlites automatically adjust the flash zoom mechanism for the best flash angle and to illuminate only key areas of the scene, which conserves power. Altogether, this technology makes the flash very handy for fill light in standard lighting and especially for backlit subjects.
With either the built-in flash or a Speedlite, you can use all of the camera's Creative Zone modes knowing that the exposure settings are taken into account during exposure given the maximum sync speed for the flash.
Flash sync speed matters because if it isn't set correctly, only part of the image sensor has enough time to receive light while the shutter is open. The result is an unevenly exposed image. When using a flash, the 40D doesn't allow you to set a shutter speed faster than 1/250 second, but in some modes you can set a slower flash sync speed, as shown in Table 8.1. Using Custom Function C.Fn I-7, you can set whether the 40D sets the flash sync speed automatically (option 0) or always uses 1/250 second (option 1) when you shoot in Av mode.
8.1 This image was made with the built-in flash capturing the motion of water dripping into a water glass. A slight blur of the water droplet moving upward provided the light streaks of motion. Exposure: ISO 1000, f/2.8, 1/250 sec., using an EF 100mm, f/2.8 Macro USM lens.
/Note In Basic Zone modes except Landscape, Sports, and FlashOff, the camera automatically fires the flash when it determines the light level is too low to produce a sharp handheld image.
E-TTL II technology also enables high-speed sync flash with accessory Speedlites that allows you to sync at a shutter speed faster than the camera's flash sync speed of 1/250 second. As a result, you can open up the lens when shooting a backlit subject to create a lovely blur to the background while shooting at faster than X-sync shutter speeds.
Tip Normally, flash is synchronized for firing the moment when the first curtain finishes traveling but before the second curtain starts traveling. However, with high-speed sync, the flash duration is extended to make synchronization possible using fast shutter speeds by forming a slit between the first and second curtains as the curtains travel.
When shooting with accessory Speedlites, Canon's flash technology allows wireless, multiple flash photography where you can simulate studio lighting in both placement and light ratios (the relative intensity of each flash unit). You can use up to three groups of Speedlites and designate a master, or main flash, and slave units that fire in response to the master flash unit.
The Macro Ring Lite MR-14EX and Macro Twin Lite MT-24EX can also serve as master flash units.
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