I think I have a Canon E-TTL issue with a specific camera body/lens combination when the flash is fired through a modifier (umbrella or softbox).
When I use my EF-S 17-55 f/2.8 lens on my 60D body, using a Hahnel Tuff TTL trigger to fire a Canon 430 EX II flash, the image is vastly underexposed when the lens is zoomed beyond 35mm. The FEC has to be adjusted to at least +1 FEC to achieve the same exposure at 0FEC when the lens is at 35mm or less.
This only happens when firing through a modifier. Direct or bounced flash works perfectly ok.
Any other lens on the 60D body works fine throughout the zoom range, and the 17-55mm lens on another body works perfectly fine throughout the zoom range, when using E-TTL flash (on or off camera). At 0FEC there is no noticeable difference between exposures at different lens zoom distances.
But with this lens on the 60D beyond 35mm the image is noticeably underexposed at 0FEC compared to the exposure at 35mm (or less) at 0FEC.
I thought the wireless trigger might be the issue, but it works fine with this lens on another body throughout the zoom range through a modifier.
Another thing that tells me the wireless trigger isn't at fault is the fact that I can mount the flash on the 60D's hotshoe with an Ezybox Speedlite attached (not recommended!) and it still underexposes when the lens is zoomed beyond 35mm. Take off the Ezybox and the flash works fine beyond 35mm.
So this only appears to be an E-TTL issue with this body/lens combo when fired through a modifier and the lens is zoomed beyond 35mm. When using manual flash the exposure is consistent from 17mm up to 55mm with this body/lens combo.
I can work around this very specific problem, but it's got me baffled and I wonder if anyone knows why it's happening?