Does zoom effect the flash, over or under expose

[Kingwing]

Greetings, can someone help, I have a Canon EOS300D and a 580ex speedlite, I also use a canon20d and a 580ex speedlite at work. I have the same problem with both cameras in that they seem to overexpose images when they are used with the 580ex in E-TTL mode.
My theory is this: The flash senses the zoom of the lens and the f stop. It sets the flash emitted light to suit. However the sensor or CCD size of the camera is not 1-1 with the zoom, ie a 50mm lens on either of these cameras gives a zoom of around 80mm. this is due to the size of the ccd and the calcualation, of lens size x 1.6 = zoom or lens size.

Is it possible that this ccd to lens effects the exposure, and if so would an adjustment on the flash help and how would I calculte the number of stops required to equal the 1.6 adjustment.

Also am I way off track

Thank you

Hank

[fotodog]

The 580ex does have a sensor that will view the lens setting (zoom) and the fstop (along with sensitivity and the metering) and adjust it's output based on that.

You may want to double check the metering and see if you're on spot or matrix, that could affect a lot of that.

Test the flash in manual modes as well to eliminate the possibility of malfunction of the unit.

Can you post/link to some photos for examples.

Thanks, man!

[Kingwing]

Thanks for that, I have tried it on manual settings and it works fine, I will have to track down how to post images, but I have adjusted the f-stop by 1/3 and 2/3 under, this seems to help, but am still learning to use this flash,
I understand spot metering but can you explain matrix, thanks

Hank

[fotodog]

Matrix metering takes an reading from multiple points in the scene and makes a calculation based on an average exposures. Canon and Nikon have the best systems for now.

Center-weighted is the old-school way of doing it where 80% of the metering is taken from 50% of the the screen (what's in the center of the frame)