Problem Using Sigma Flash on 40D - Overexposure

[steve_byland]

I am using a Sigma 500 DG Super flash on a Canon 40d for fill flash in late

afternoon on birds at a feeder. If I shoot 2 shots in quick succession, the

first shot comes out exposed correctly, while the second (which may be taken 2

or 3 seconds later) is totally overexposed. I would have expected the flash

strength to fall off rather than blowing out the whole photo. If I wait 5 or 10

seconds between shots, the exposure is fine.

 

I have tried several different modes (manual, AV), different exposures on the

flash (-2.0 to +2/3). I am shooting in lower light and using shutter speeds

well below the synch of 250th of a second. I have tried varying the speed and

f/stop combinations (shutter speeds from 1/50th to 1/200th).

 

I can usually tell right away when the flash has fired too hot, as I hear the

high-pitched whine after the photo of the flash. Any help would be much

appreciated!

[wilbur_wong]

Does the Sigma flash utilize the ETTL procedure of the Canon? There should be a preflash which is used by the TTL sensors and processed to determine the proper main flash strength.

 

Maybe the pre-flash doesn't fire on the quick succession possibly due to an exhausted capacitor? As an experiment, try the same as you are doing but boost film speed to 1600, or something else to force the flash not to fire at full power.

[bob_osullivan]

Or just wait longer between shots and see if that does it.

[PuppyDigs]

I wonder if the ROM is up to date?

[mark u]

I think your problem should be addressed to Sigma. It would be interesting to know their response, and whether they believe that their newer 530 DG Super model is better behaved with a 40D.