canon did it again: 580ex II

Started Oct 29, 2007 | Discussions thread
OP cityphotog Senior Member • Posts: 1,337
sure

here's the thing:

the flash has three modes:

1. E-TTL
2. Manual
3. External Metering (auto thyristor)

the camera gives the info about the shutter speed, ISO, aperture and zoom (of the lens) to the pins of the hot shoe. The flash gets the info and elaborates. All happens real fast and the communication is always on (between the flash and the camera).

in e-ttl the flash will adjust the intensity automatically
in manual it will fire full power and you must compensate (manually)

in external metering (auto thyristor) the flash will operate automatically only if you shoot a 1DIII (and the 40D) but "manually" with any other camera : you will need to "tell" the flash the ISO and aperture (both values the flash already has).
that's why I was laughing at the canon marketing for this nonsense.

why I like the external metering (auto thyristor) more than e-ttl:

because e-ttl is basically a "spot" metering for the flash, since links the metering to the focus area selected and active at the moment of the pre-flash. E-TTL II (a function of the camera, not the flash) still takes the focus area under consideration, but extends the metering to a larger portion, compensating if there will be a big difference in light within.

the external metering (auto thyristor) couldn't care less of the focus area active and gets the metering from the light of the flash itself returning from the scene. during the trip from the flash to the target and back to the flash. It simply "cuts" the emission when the scene is covered. It calculates the average with prevalence on the center. And it works beautifully pretty much all the time: direct flash or bounced, it doesn't matter.

now only the 1DIII and 40D owners can benefit from it: all the others need to set the aperture and ISO manually. Again the funny thing that the flash knows both already.

LOL

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