Indexicality
The property of a photograph as a physical trace of light reflected from a real object, rather than a depiction made by hand.
In depth
An index has a causal connection to what it represents, in the way a footprint or a fingerprint does. This is the basis of photography's evidential authority — the claim that something was there. Digital capture and generative image-making unsettle it, which is why the concept has become central to debates about photographic truth.
Examples in practice
A photograph of a footprint and the footprint itself are both indexes of the same passage.