Originally Posted by soupman
I have experienced many AF problems when my AB-800's are at a low setting. The modeling lights are doing what they are supposed to do...just too dim for the AF on my Canon EOS Rebel XT. I have read that another hot light as RFS suggests will work...but have not tried that. As I am using reading glasses now, I find it difficult to manually focus via the viewfinder too.
You can change that setting on the ABs so that the modeling light does not track the power setting of the flash. This leaves the modeling light at full power.
There is usually a diopter adjustment on SLR cameras that lets you adjust the viewfinder and this is useful for people who wear glasses. I use reading glasses, but I can not clearly see through the viewfinder while using them. So I adjust the diopter setting so that I can see the viewfinder perfectly without the reading glasses (but I have to put the reading glasses back on to chimp - etc).