I'd say in filmmaking, no matter what role you're doing, it behoves you to know as much as about the whole process as possible, even if you plan to specialize in one particular field. The director's job is to be able to communicate what he/she wants to see in the lighting and image, but then it's up to the cinematographer to use his/her knowledge to do it in the most beautiful and effective way possible. ultimately it's a team effort!