Football is a deep enough sport to where even the most involved fan still always has something to learn. A simple thing I've ALWAYS wondered is: how do they determine who wears what helmet variation, especially the face mask? Some of it is obvious, a lineman is obviously going to need more to keep prodding fingers out of his eyes whereas  a  QB needs vision. given that consideration, there is still a wide variation in the types and styles worn. Why, for instance, does peyton manning wear his odd looking mask and helmet while Casses wears a simple mask of two horizontal and two vertical bars?

Is is head shape? preference? what's available? something else?

just a point of curiosity i've always had.