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<blockquote data-quote="WayneF" data-source="post: 462279" data-attributes="member: 12496">FWIW, old &quot;FILM&quot; TTL was quenched off by the camera when it was seen metered (in real time) as sufficient. Individual metering of multiple flash was impossible then, as was a wireless concept like the Commander.But digital iTTL is instead preprogrammed for the power level (which is a duration) by the camera after metering the preflash (individually). This involves CLS communication (hot shoe pins or Commander).I'm making this next part up myself, but obviously the camera does not know the power capability of the various flash units (no concept what 1/4 power might be on that flash.)&nbsp; So the camera can only program TTL as relative power, like &quot;Now do 5.3 stops more level than that TTL preflash I just saw from you&quot;.The camera system does the TTL metering, and a flash is just a flash (it just flashes), but it does have to understand the instructions in a consistent way.Making this part up too, just a hunch, but there does seem to be a difference in the way the 3rd party flashes and cameras report and treat the default TTL BL power reduction too.</blockquote>

[QUOTE="WayneF, post: 462279, member: 12496"] FWIW, old "FILM" TTL was quenched off by the camera when it was seen metered (in real time) as sufficient. Individual metering of multiple flash was impossible then, as was a wireless concept like the Commander. But digital iTTL is instead preprogrammed for the power level (which is a duration) by the camera after metering the preflash (individually). This involves CLS communication (hot shoe pins or Commander). I'm making this next part up myself, but obviously the camera does not know the power capability of the various flash units (no concept what 1/4 power might be on that flash.) So the camera can only program TTL as relative power, like "Now do 5.3 stops more level than that TTL preflash I just saw from you". The camera system does the TTL metering, and a flash is just a flash (it just flashes), but it does have to understand the instructions in a consistent way. Making this part up too, just a hunch, but there does seem to be a difference in the way the 3rd party flashes and cameras report and treat the default TTL BL power reduction too. [/QUOTE]

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