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Canon EOS Digital Rebel XT Digital SLR
Canon EOS Digital Rebel XT Improved Flash
The Canon Rebel XT also features the next step in the evolution of evaluative through the lens exposure control, E-TTL II. Though compatible with all Canon EX-series Speedlites, this "smarter" new E-TTL II system helps ensure that, even in situations with highly reflective subjects or backgrounds or when shooting in most other similarly difficult lighting situations, the image will be optimally exposed.
The E-TTL II system compares the ambient light with the reflected pre-flash off the subject reported in all 35 metering zones and selects the areas with a small difference to be weighted for flash exposure calculation. This system also helps eliminate or underweights areas with large differences, recognizing them as an extremely reflective object in the background or a highly reflective subject.
The XT supports the new functions of Speedlite 580EX, including auto zoom for picture size and color temperature data transmission.
E-TTL II Autoflash
E-TTL II takes effect when the built-in flash or an accessory EX-series Speedlite is used. E-TTL II does not assume that the AF point in use covers the main subject. Instead, when the shutter button is pressed completely, ambient light is measured just before a pre-flash fires. For each metering zone, the ambient reading and the pre-flash reading are compared.
Canon Speedlite 580EX |
Areas having a large difference between ambient and pre-flash readings are selected and analyzed. Extreme differences suggest a highl reflective or specular object; these areas are eliminated from calculations, avoiding underexposure. When a lens providing distance information is used, this data is also considered in determining the presence and location of a high-reflectance object. The selected pre-flash readings are added and averaged and the output of the main flash is calculated. Even if the subject changes position, reflectance or size, the flash metering remains highly accurate and stable.
Improved Built-in Flash
The EOS Digital Rebel XT's built-in flash has a guide number of 13/43 (ISO 100, meters/feet), the highest in its class as of February, 2005. The built-in flash has greater coverage, now handling a 17mm focal length, and the flash head has been raised 5.5mm higher than the Digital Rebel, reducing red-eye and eliminating blockage of flash illumination when using the EF-S 18-55mm and EF-S 17-85mm zoom lenses.
User-adjustable flash exposure compensation up to +/- 2 stops in 1/3 or 1/2 stop increments has been added. Second-curtain flash synch has been added. Flash exposure bracketing, FEB, is now effective when set with the 580EX, 550EX, MR-14EX or MT-24EX Speedlites. Modeling flash is enabled with the 580EX, 550EX, 420EX, MR-14EX and MT-24EX Speedlites. In the Creative Zone modes, press the depth-of-field preview to fire at 70 Hz for 1 second.
The Canon EOS Digital Rebel XT camera supports wireless flash with these same 5 flashes and the ST-E2. Three-group slave control, a flash output ratio control, FEB and modeling flash (with flash output ratio) are all enabled. (The ST-E2 supports up to two slave groups with flash output ratio control, but not FEB or modeling flash.) Note that the 420EX can function as a slave only, while the MR-14EX and the MT-24EX can serve as the master unit only.
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