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I used to hear my mentor (Hollis Frampton) say that a photograph was a slice of time and space. I'd like to expand upon his remarks. To say that it is a slice of space and time is to say that it is essentially linked from the photograph to that time in space and time. This relationship between the two is indexical in essence. This link between the two is so strong that we can get fooled by tricks (i.e. photoshop). In a painting this would just be considered part of the painters "artistic vision" but with a photograph we expect that the image bears some necessary connection to reality.
THe downside of this linkage is that the other two main ways of linking a piece of art to the world we see, taste and feel is either iconic or symbolic. I will claim that no photograph has the iconic nature of say the Mona Lisa or the symbolic nature of an old masters painting. When photographers try and imbue a photograph with symbolism it turns out to be a strange duck. Look at the works of Josef Fischnaller who imitates old masters paintings. Instead of the symbolic aspects being in harmony they appear forced and alien because we seek to link that picture back to a time and place.