Just noticed this the other day, my 17-55mm lens hood is huge and it partially blocks the red patterned focus assist beam of my 430EX flash. i haven't noticed any difference in test pictures with the hood on and off. but i am wondering if this will be a problem in real world application? i'm sure other people have come across this problem (if it's even a problem) with lenses like 24-70, 70-200 etc. Just wondering if i should be concerned or not. thanks!