III. DISCUSSION
13
E-TTL II Autoflash
The AF point can be selected manually with the cross
keys on the back of the camera, or with the electronic
dial. The user can avoid pressing the AF point selection
button before using the cross keys to select an AF
point by activating Custom Function 1-4. The AF point
selection operation with the cross keys is similar to the
EOS ELAN 7N. With the electronic dial, AF point selection
is the same as with the EOS Digital Rebel camera.
To enable manual focusing with the focusing ring, the lens focus mode is switched to M
or MF (with Canon EF and EF-S lenses). When focus is achieved, the focus confirmation
icon and superimposed display light up.
The AF-assist beam is a series of stroboscopic flashes from the builtin flash or a near-IR
patterned LED with a compatible accessory Canon Speedlite. With the built-in flash, the
working range of the AF-assist beam is approximately 4 meters/13.2 feet at the center
and 3.5 meters/11.5 feet a the other 6 AF points. Firing stops when the focus detection
confirmation signal is detected.
The EOS Digital Rebel XT camera incorporates the
E-TTL II autoflash system used in the EOS-1D Mark II,
EOS-1Ds Mark II and EOS 20D. It is truly remarkable
that such a sophisticated algorithm has been
included in a camera at the XT’s price point. 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.
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
Automatic AF point selection
Automatic AF point selection
[With cross keys]
[With main dial]
Automatic
AF point selection
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