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...Since ETTL flash exposure is based on measuring a preflash and extrapolating the required output from that, putting a modifier on the flash can affect the accuracy of the flash metering....
Is this a contradictory statement? If the camera, in E-TTL, determines exposure based on a pre-flash, shouldn't the modifier automatically be factored in? If the modifier reduces the pre-flash by let's say one stop, the camera will see one stop less from the pre-flash and thus increase output by one stop, no? I thought this is exactly how E-TTL works.
One would think that it would work that way, but experience has shown that it doesn't. For some reason, modifiers mess up ETTL on many Canon bodies.
With my Vivitar 285HV I don't have that problem, because it's reading the actual reflected light from the subject during exposure. Modifiers aren't a problem, because I have an extension cord for the auto sensor so I can move it to the camera's hot shoe. The 580EX II has such a sensor, but it's fixed in place, so many modifiers will block it.