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Old Feb 16, 2006, 1:16 PM   #1
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OK flash gurus, I need some help with this one. I have a 20d and use a Sigma 500 DG Super flash. Now, here's my question:

Assume the camera is in manual mode at a specific ISO, aperture and speed (for sake of argument assume ISO 400, 5.6 and 1/60) and flash in ETTL mode.

Now, when I use my Canon 28-135 lens I constantly have to use more FEC to achieve proper exposure. If I switch to my 17-40 or 50mm 1.8 or Sigma 70-200 2.8(and still keep the same ISO 400, 5.6 1/60 settings) I have to dial down the FEC. It's fairly consistant for me so I know if I switch lenses I need to adjust the FEC, but I can't figure out why? I didn't think the max POSSIBLE aperture of a lens had any bearing on how the flash would meter. I also haven't tried taking the flash out of ETTL mode and instead setting it in manual. If I do that I would expect exposure to match regardless of which lens I used. Which would tell me the flash in ETTL is for some reason determining it needs LESS power with the 28-135 than with the other lenses. Can anyone help educate me on why this happens?

Thanks for the help.
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Old Feb 16, 2006, 1:21 PM   #2
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Just guessing here but wouldn't where the lens focuses determine distance to the object and flash power?
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Old Feb 16, 2006, 1:26 PM   #3
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bobbyz wrote:
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Just guessing here but wouldn't where the lens focuses determine distance to the object and flash power?
Assume same subject and distance. Also with ETTL II of the Canon 20d, metering is evaluative and not weighted to the active focus point(s) like ETTL - of course based on my limited understanding of how ETTL II works. I could maybe understand if I was shooting wide with the 17-40 and thus it had a larger area to meter but the 50mm 1.8 actually shows the highest swing in FEC adjustment. Or maybe I'm just going crazy - that is always a possibility :?
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Old Feb 16, 2006, 2:09 PM   #4
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I think what bobbyz meant was the distance info coming from the lens and not the AF point). The E-TTL II uses this info over the E-TTL which do not factor in the distance (also notice that some older lenses) do not provide this data...

Is it a 50 f/1.8 I or II?
(my 50 f/1.4 and 85 f/1.2 don't provide this distance info to the camera either)http://bobatkins.photo.net/photograp...faq/ettl2.html
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