Hi, I've just got started to do some serious photography and am now buying equipment. I'm looking for heads to match a Gitzo G1228 tripod. I came across some terms which I don't quite understand, but I believe they are critical in selecting heads. What does "Tripod Mount Thread Size" mean? Following are some head specs that I got, what do they mean? Thanks a bunch! "Tripod Mt. Thread Size 3/8"-16" "1/4"-20 camera mounting screw"