Is this photograph "authentic"? It presents, more or less what one would see if they went to the Con mine in Butte, Montana. At the same time, it doesn't.
This image started out as a RAW file in my camera. I took it out and did some serious editing, (removing the three trestles behind the headrace and editing out the foreground), and converted it to a "drawing" using "pen sketch" filters in PhotoShop. Then edited some more. I did not do this to deceive the viewer, I did it to get my interpretation of the headrace in front of them.
My goal was to include enough "real" information in the finished product to make it recognizable for what it was. At the same time, I wanted the image to convey what my interpretation of the subject was - stark, industrial, crude. This makes it art. Art I couldn't have done in a real darkroom. The image is fundamentally changed, although I don't think it makes it any less authentic.
Since my medium was an image captured by a camera, I think - no matter how modified the image was - in the end, it is still a photograph.
As far as poeple "believing" that an image is an exact conveyance of what the photographer saw, I don't think that is, necessarily, true. Look at "picture postcards." I've seen hundreds and in almost every case, standing and looking at what they portrayed, tried to figure out how they got that image. "Picture postcards" are, allegedly, representational. They are representational of what the photographer saw when he took the picture.