Originally posted by top-quark Huh? The metering negotiation between body and flash should happen in milliseconds and shouldn't be noticeable. Sounds more like red-eye reduction to me.
Have you actually measured it? Or have you found the spec somewhere?
The time between the preflash strobe and the main flash is actually noticeable. Some people (my son included) react to the preflash strobe and thus likely close their eyes during the main strobe. For this reason, I don't use p-TTL mode for group photos.
In my non-scientific guesstimation, the time between the p-TTL preflash strobe and the main strobe is in the neighborhood of 1/50 sec. Note the sequence:
1. Flash fires preflash strobe.
2. Mirror raises.
3. Leading shutter
fully opens.
4. Flash fires main strobe.
5. Trailing shutter closes.
How long do you think it takes for (2) and (3)?
Do this simple experiment: setup an optical slave flash, one that is "dumb" (can not ignore the p-TTL preflash strobe). Using the built-in flash to trigger the slave, take a photo in front of the mirror, with both the camera and the slave flash visible in the mirror. Check the photo to see which flash(es) fires during exposure.
Last edited by SOldBear; 04-10-2013 at 02:31 PM.