On Wednesday, June 18, 2003, at 11:49 AM, Alex Z wrote:
It is known that ETTL is heavily biased on active AF point producing precise flash exposure for particular piece of the subject that is covered by tiny AF point (out of 45 of 1V/EOS-3 and their digital siblings). So far I was mostly impressed by the precision of the ETTL flash exposures, especailly for fill-flash that is usually recognized as the most difficult for camera system, however, the case raised on Rob's Forum may impact the reliability of ETTL in certain situations. The issue is more related to shooting subjects featured by high contrast sweep such as weddings (bride+ groom portrait, for instance). In this case, I would aim the particular AF point on subject's eyes to ensure proper flash exposure for the face, but then may end up with severely overexposed bride's dress (that is usually shiny white) or underexposed groom's black suit. In such case, more balanced (perhaps more averaged) flash metering would be desired (aka, center-weighted averaging). This way the are chances more contrast range will be preserved in the image. Word of caution: I haven't yet tested this myself, so just passing the info.
The cure for this potential problem would be to configure CF 4 (available on EOS-3/1V/1D and 1Ds) to define the * button for AF while AE is still triggered by release. Then, once AF is achieved and confirmed, releasing this button just prior to releasing the shutter would force ETTL to measure over entire 45 sensors AF field producing some kind of averaging over the entire area. This is supposedly to take into account the wide spread pf contrast over the image and is expected to produce more balanced flash exposure preserving more or less overall image contrast range.
Actually, AE lock doesn't lock the flash exposure. That is handled by a separate "Flash Exposure (FE) Lock" button. If you are using a D30, D60 or Elan 7, the * button can either be used for autofocus or FE lock.
According to Canon, in order to obtain a center-weighted flash metering pattern, the solution is to put the lens in MF mode. Also, the new Canon EOS 10D has a center-weighted flash metering pattern.
In a high-contrast situation you've mentioned (bride with white dress and groom in black tux with white shirt), you'd shoot low-contrast "wedding/portrait" film such as Fuji NPH or Kodak Portra NC, or set the contrast to "low" on a digital EOS body. I've never had an problems with whites on clothing blowing out on Fuji NPH or Kodak Portra NC.
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