Walton (1984) writes of the photographic medium as being something of an enigma that both represent and does not, the nature of what is observed. For me the discussion of the apparent and the perceived is a fascinating subject even though quite philosophically abstruse. The situation as Walton points out is that we perceive the data of our senses and not the thing in itself (1984) 
I find this discussion a somewhat edifying and at the same time elusive if looked at too directly; a bit like what happens with our senses in general although often unaware: we tend identify with the resultant factor of an object rather than the causal source. A photograph is a representation of something in the external world that has a light quality in terms of intensity, direction etc it is not the thing in itself. However, because of how we are/were trained to see photos, we took the image to be the thing in itself.
Walton argues that photographic realism in a mis-perception. Arriving at the idea of realism in photos in an obvious conclusion that rests on what is seen but at the same time and as I know from my own experience there are many levels of interpretation that I can give to any one photo that is based in looking at an object.
- Walton, K. Transparent pictures: On the nature of photographic realism (1984) The university of Chicago.