In photography, there is always one person who is present yet never seen. That person is, of course, the photographer. He takes all the pictures, but never appears - unless someone else has a camera or mirror.
In the namesake gospel, John’s name never appears. Yet, the intimate perspective of the narrator suggests someone close to Jesus, which we know to be John. In addition, the other gospels make it clear that John was present at those events.
By process of elimination, we can be fairly certain that John is the chief photographer of this gospel (figuratively speaking).