Hi all,
I just got my 550EX flash unit and did a couple of tests to demonstrate how E-TTL works. If you're still having problems figuring out how to expose properly with E-TTL, this may be helpful to you:
1) Focus using centre point, shoot.
2) Focus using centre point, recompose, shoot.
3) Focus using centre point, FE lock, recompose, shoot.
4) Compose image, focus using middle-right point, shoot.
5) FE lock using partial metering circle, compose image, focus using middle-right point, shoot.
6) Set focus point to middle-left, FE lock using partial metering circle, focus, recompose, shoot.
All shots were done with the Tamron 28-300XR at 50mm, 1/125s, f/8.0, ISO100, Daylight WB. No processing done to the pictures other than resize and USM. The focus point is the middle of the subject's two eyes (the centre point, however, is large enough to cover both eyes). The black background is perfect for testing E-TTL exposure - if anything is wrong then it will show up as a horribly over-exposed image.
Conclusion:
Results are in agreement with the E-TTL and Canon flash FAQ at //www.photonotes.org - E-TTL meters on the active focus point. Also, pics #5 and #6 show that if there is no active focus point, using FE lock will meter on the centre/partial point, even if you have set your focus point to one of the other six points.
I hope this clears some of the confusion (and exposure issues) that people have been getting with E-TTL flash metering.
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