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 I have anything to give you through camera, it must be of myself. โฆ A gnawing burns inside โฆ to make something of myself worth giving.
Minor White