Sto-fen diffuser - under exposure only when shooting straight on

Why do I get under exposure when shooting straight on with a sto-fen on a 580EX II or 430EX II in E-TTL mounted on a 1Dmk3 or 7D in P, Av and Tv? Note that if I use FEL the exposure is set correctly.


This issue only occurs shooting straight on. As soon as I bounce the flash head in any direction then the exposure is fine.


I thought using E-TTL that I shouldn't need to worry about this?? Any insight and help you can give as to why this is occurring (and how to fix the issue while still shooting straight on without FEC ) is greatly appreciated.


Thanks,
A

Hi AMR. I know that when shooting in those modes the flash is typically only fill flashing. So, depending on the scene and metering, that is how the camera interprets how much flash to throw. When you bounce, it is exposing as if there is no flash(at least not fill), and adjusts accordingly. That is why I mostly shoot flash in manual only, unless I do hi-speed or rear-sync. Scott Kelby explains in one of his books(?), to adjust your exposure in manual, with flash off. Get your settings, then turn the flash on, with your f-stop and shutter value the same, then compose and shoot. Seems to work in most instances. I don't have the technical know how to explain it better...I apologize. I, too, use a Sto-fen almost 100% of the time, and actually prefer using it with the flash head at a 45* angle therefore getting somewhat direct, but also bounce at the same time. Hope this sorta helps??

I don't have an answer for you, but I'll throw in the observation that when you have your flash pointed directly (diffuser or not), the camera-to-subject distance is used in the flash exposure calculation, and the flash head zooms to the appropriate focal length (you can see the focal length on the flash's LCD). But, when you angle/rotate the head, the flash head no longer zooms. Not sure if that's a factor, but it is one thing that substantially changes between straight-on and bounce.

When using this type of light modifier set the flash zoom to a fixed setting. I have not noticed this with my flash but I have noticed a difference in color balance.


Mark