E-TTL and ISO

I was doing some ISO noise testing yesterday with my DRebel and 420ex.

I discovered that most of my shots were WAY over exposed.
Then I realized, it was all the shots ISO400 or greater.

I never realized that E-TTL doesnt take into account the ISO setting.
Atleast it doesnt on my setup.
Does this sound right?

What aperture and shutter settings and shooting mode were you using? Just curious.

That's not right.

Were you working within the shooting range of the flash for ISO 400? If you are too close, the flash may not be able to cut power sufficiently to avoid overexposure.

Another thing to consider is the contribution of ambient light. If you're shooting at the typical 1/60 sec, f/4, the resulting exposure could also be overexposed.

ETTL is good but it can only reduce the flash output to a certain point. As you go up in ISO you increase the Guide Number of the flash. You need to be sure that for a given ISO you are beyond the minimum distance for a given f stop and Guide number. The 420EX has a minimum distance of 0.7M at ISO 100. At ISO 400 the Guide Number increases by a factor of 2. That means you need to be at least 1.4 Meters away.
Scott

Thanks for the info.
I MAY have been near the 1.4m.

I will do somemore testing.

High Speed Sync?

Just in case you are in AV mode, check also the shutter speed. If metering requires more than 1/200 shutter speed and the flash was not set to high speed synch, the resulting picture will be over exposed.

Well THAT explains a few things! I wish I had known that a few days ago.

Thanks!

You're welcome. Just remember that when you go from ISO 100 to 200 the Guide Number increases by a factor of 1.4, and when you go to 400 by a factor of 2. The same is true in reverse(100 to 50 and 50 to 25) in case you use film.
Scott