ah fair enough..bright torch on area you want to focus,once its in focus with auto focus switch it to manual..i used to try this back in the day with duller torches and had trouble,but p7 or similar Chinese bright ones will work wonders for you!I was using different stops ranging from f11, f8 and f4.5 usually. I think my problem though is relying too much on auto focus. I was using my 6d down there which is great for low light, but in pitch black the AF is no really any better than my entry level Sony.
Sorry to clog up your thread with some words of advice