ISO - ETTL effect

[Derek Smith]

for those with a 10D and a Speedlite 550X, try this:-

Set ISO to 100 and take a flash shot using ETTL
then
Set ISO to 1600 and take the same shot again using ETTL

When I tried this, the faster film setting completely screwed the exposure, overexposing by several stops.

If anyone else can reproduce this effect, then its not the ghost of Cannons Past in my firmware - probably just a bug that Cannon forgot to programme in.

Any thoughts?
Derek

[bluebomberx]

I have had the same problem with higher ISOs. I'll look into testing it further.

[CyberDyneSystems]

Does the Ettl not take ISO settings into account?

Do you have to use ISO 100?

[Arnie]

swith your 550ex to High Speed Sync if your speed is higher than 200.

i encountered this problem in AV mode, full open, and very bright ambient lighting and my 550ex not set to High Speed Sync

[leony]

The problem is usually (from my experience) is that 550EX is not set to high-speed sync (FP flash). This causes the camera to shoot at 1/200 sec. or slower, and so if your lighting and appereture require a shutter speed of say 1/500 sec. (which is not uncommon with high ISO) you will get over-exposed images.
Another reason is that you're too close to the subject, and even at the smallest power 550EX is still to strong and will over expose.

~ Leon.

[bluebomberx]

I haven't tried HSS yet and will try it soon, but my problem stems from very low ambient light. Shutter speeds are well below 1/200 in my case.