Hi Stefan,
Using EOS3 & 1 or 2 550EXs, occasionally I noticed quite annoying
under-exposure when I had one 550EX on camera.  I traced this to specular
reflection from shiny fabrics fooling the metering system.  (It seems as
though it doesn't require a big area of shiny stuff to give problems.)  Test
shots taken with the 550EXs bounced into brollies were beautifully exposed.
Examination of these slides showed no problems from specular reflections.
You could try setting TTL on the 550EX (rather than E-TTL) where the
sensors, working off reflections from the film, should be less susceptible
to this problem.
This might be another reason why press photographers nearly always diffuse
their guns.  I know they want a softer effect, but as a side effect they
might get more reliable exposure as well.

M Stewart              Milton Keynes, UK
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I can backup what Mr Stewart has said above !
I too found that my EOS3 + 550EX combination under exposed a little from
time to time, And by doing a little experimentation was able to track the
problem down to specular
reflection when I used an atomiser spay (To fake dew drops) on my subjects
(flowers and fungus)
I now use a handheld light/flash meter when I photograph this WET kind of
subject (Although I have from time to time forgot to add some exposure
compensation because of close focusing with my 180mm macro lens)


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