There are as many ways to look at, to apprehend, a photograph as there are people, at least. Still, in broad strokes, we can think about how people look at pictures.
You can look at a photograph purely as a representation of some subject. Look, a picture of grandma. Susie, standing in front of the Eiffel tower. Here, the criteria for judging are pretty much whether or not the picture is sharp enough to make out the subject, and perhaps whether or not the picture "looks pretty" in some sense. Is it a flattering picture of Susie, or does she look awful? We all look at pictures in this mode, much of the time. Most people look at most pictures most of the time, like this.
Many kinds of photographs really admit almost no other mode. Macro photographs of bugs, pictures of the moon, snapshots of our lunch. Most of the kinds of pictures I dislike, it turns out, are of this sort. We could examine them in the modes discussed in the sequel, but it would be a stretch, and we generally do not. It's also not clear what would happen if we tried. Sometimes these are technical tours de force as well as being a representation of an interesting subject, which adds a little extra something, for the camera types.
Other photographs are a more thoroughly evocative experience. They remind us of something, they make us feel a little about something, they connect us to a larger experience. Beautiful pictures of sunsets might have this effect. A good portrait might. News photographs connect us to current events, make us experience it in a way that words cannot.
Lastly, at the other end of the spectrum, there are Art photographs, whatever that means. These things are all about the emotional response, all about making us feel or think or understand something. They ask probing questions, tell us potent truths, they show us things we barely imagined. At least when they're working well, they do.
Photographs in general tend to be apprehended in a large part by their resemblance to other photos we have seen, often photographs we have seen and apprehended in that same mode. Grandma looks so much better this summer than last. That piece reminds me of that one photo of Giacommelli. I liked the portrait of Bernstein better. The situation in Egypt looks so much like it did last year. Art is compared to Art. Snapshots of birthday parties are compared to other snapshots of other birthday parties. In this age of a trillion pictures, it is almost impossible to look at a picture without feeling the influence of a massive, albeit largely unconscious, collection of other pictures we have seen.
Some photographs benefit by the comparison and the connection to the other pictures. Pictures of grandma fit into many places, not least into a narrative of grandma's life told, in part, through pictures. Pictures of sunsets are arguably supported and sustained and improved by the fleet of other pictures of sunsets we hold in our minds, as well as by the actual sunsets we have seen.
Other photographs, at other times, are ill-served by this connection, by this massive army of other pictures. There is a strong tendency to judge, in particular, Art, in terms of how much it resembles other Art. If the point is to say or show or ask something new, we find ourselves in a bit of a bind. The photographs, especially when seen by themselves, tend to represent not a new message or idea at all, but an endpoint in an evolution of images. It's as if Cindy Sherman shot landscapes in the style of Ansel Adams. It's a Westonesque take on Mapplethorpe. The picture itself is lost in the maze of comparisons and influences, real or imagined.
Thus it is that the portfolio becomes more and more important as more and more pictures are made and seen. A single picture has almost no chance of standing alone. It must be a truly remarkable picture to not be simply seen as a re-mix of previous artists and pictures.
A portfolio, though, has a much better chance of standing alone. The influence of the artist becomes more apparent through repeated stylistic elements. What seems an accident, marring the Weston homage, in one picture becomes the stylistic signature when viewed in a portfolio. The Adams influence in one picture might become a mere accident when the whole portfolio is seen -- or just as well -- the portfolio can be seen as legitimately influenced.
A portfolio of pictures, well designed, provides clarity of vision. I think an argument can be made that the age of the single photograph is over, and I think I have just made that argument. Shooting a good portfolio is leagues harder than shooting a good picture. Is that good or bad?