hey, i use a canon xsi too, and i find that when i use manual mode, the exposure sets by itself. i can't find a way to lock it down. maybe you could try using Shutter Priority or Aperture Priority? on those modes, the exposure can be locked down.
did you figure this out in the end?
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Canon EOS 450D / Rebel XSi
18-55mm, 55-250mm
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