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The new Speedlite 550EX is powered by 4 AA-size (LR6) alkaline
or KR15 Nicad batteries. The 550EX also has a new external battery power source,
the Compact Battery Pack CP-E2 that is powered by six AA-size (LR6) alkaline batteries
for faster flash recycling time. Lithium AA-size batteries can be used for both the
550EX and the new CP-E2 pack.
The E-TTL automatic flash output from the 550EX is possible in the Program AE, Shutter
speed-priority AE, Aperture-priority AE and Manual Exposure modes. It has a guide
number of 36 (35mm, ISO 100, meter). It has all the features of the Speedlite 380EX
that was introduced with the EOS 50/50E cameras, like the E-TTL flash output, High
Speed Focal-Plane (FP) Flash and Flash Exposure (FE) Lock. What's new are the two
extra features of its own, bounce flash for vertical format at the 180° position
and the E-TTL Wireless Autoflash System.
The E-TTL Wireless Autoflash System works in both the E-TTL and Manual flash outputs.
Both modes worked by using one 550EX attached to the EOS-3 as the master unit to
trigger off one or several more 550EXs that are positioned near the main subject
without the necessary wire connections needed previously when shooting using the
older EZ or other EX series of Canon Speedlites.
The most remarkable feature of Canon's new Wireless Autoflash System is that you
can set the flash output ratio like A: B on up to two groups of flash unit. The broadcast
ratio range from 1: 8 to 8: 1 enables versatile light and shadow control over your
subject. This feature is available when you use either the Speedlite 550EX or the
Speedlite Transmitter ST-E2 as master unit to be mounted on the EOS-3.
With the former as the master, you can also add flash group C as accent light with
which you can set the flash exposure compensation amount on the master unit. Group
C is independently controlled beyond the main subject's flash exposure.
The 550EX can be used with the EOS-1N and EOS 5 cameras but without the benefits
of its E-TTL, FP and FE Lock exposure outputs. When used with the EOS-1N, the 550EX
will revert to normal TTL flash output in all the shooting modes. The Wireless Flash
System, however, does not allow both the E-TTL or TTL automatic flash output when
used with the EOS-1N or EOS 5 cameras.
You need an incident flash meter to obtain the optimum flash output level, divide
it by the number of flash units you use, and set the result on your master unit.
Every unit fires according to this preset output level on the master, which results
in appropriate flash exposure in total.