« on: October 26, 2012, 02:41:56 PM »
It would appear to me that you are getting the correct results. Unless you change something other than just ISO, it is extremely unlikely that a speed light can possibly expose all those shots correctly.
The EX580 II has a minimum power output of 1/128th of its full power. (I don't know what the minimum power output ratio of the EX550 is, but I would be suprised if it was significantly wider than the EX580 II) The only way that a SpeedLight would be able to correctly expose each of those ISO settings is if the shot at ISO 100 required full power to correctly expose, only then would the 12800 ISO shot be correctly exposed at the minimum flash strength of 1/128th power.
I expect that in the situation of your first set of test shots, the flash is hitting is minumum power output limit at around ISO 400. Because the flash cannot fire any weaker than that, any shot with a higher ISO will be over exposed.
With your second set of test, you decrease the amount of light that your camera could see by changing the aperature. With a 2 stop change in apperature you see that shots begin appear over exposed at an ISO 2 stops higher. Compare ISO 800<->3200, 1600<->6400, 3200<->12800 and notice how similar they are.
You can confirm this by testing with a smaller aperature. If you use a small enough aperature, your high ISO shots should be correctly exposed, but your low ISO shots will likely be underexposed. If you are still in doubt, set the flash at its minimum power and find out what ISO setting correctly exposes that flash power.