While we cannot describe its appearance (the equivalent), we can define its function. When a photograph functions as an Equivalent we can say that at that moment, and for that person the photograph acts as a symbol or plays the role of a metaphor for something that is beyond the subject photographed.
Minor WhiteIf all your life means to you is water running over rocks, then photograph it, but I want to create something that would not have existed without me.
Minor White...insight, vision, moments of revelation. During those rare moments something overtakes the man and he becomes the tool of a greater Force; the servant of, willing or unwilling depending on his degree of awakeness. The photograph, then, is a message more than a mirror, and the mans a messenger who happens to be a photographer.
Minor White