580exII auto zoom question

I'm having an argument with my camera/flash-and I'm losing. Normally, the flash is set to auto zoom and works just fine on my 50d. Tonight, I put a 100mm f2.8 on and the auto zoom says it's an 80mm. The 50 f1.4 worked fine and so did a 17-85mm lens. Any ideas or suggestions?

are you in manual zoom? you should set your zoom to the Square box with faded the faded logo and not the black logo.

all else fails.... rtfm

Im Sasquatch wrote in post #9494289
=I'm Sasquatch;9494289]I'm having an argument with my camera/flash-and I'm losing. Normally, the flash is set to auto zoom and works just fine on my 50d. Tonight, I put a 100mm f2.8 on and the auto zoom says it's an 80mm. The 50 f1.4 worked fine and so did a 17-85mm lens. Any ideas or suggestions?

If the flash is in auto-zoom mode, then the flash will automatically set the zoom to 80mm because that is the narrowest zoom position it can go to without going any narrower than the lens itself. The next zoom position is 105mm and if you put the flash in that position, there's a possibility that you'll get vignetting.

C2thew-thanks for the reply. It was in auto zoom. I've read the manual a few times and I think I feel a little dumber after each read. I freely admit to being incredibly ignorant when it comes to using a flash, but I'm trying to learn.

PacAce-thank you! I didn't realize the flash jumped (for lack of a better term) from one length to the next. It didn't occur to me that it wouldn't adjust for each major focal length. Your explanation made perfect sense to me. There isn't anything wrong with the flash-I just have a lot more to learn.

I appreciate all of the help/clarification.