In my last post, I identified the index as a sign with a physical connection to its referent; which in photography theory is generally thought to be the imprint of light left on light-sensitive material. I also noted that for many theorists – Roland Barthes chief amongst them – this physical connection to the referent is the defining, or essential, quality of photography. What then is to be made of the numerous claims that digital photography does not share this essential quality?
There are a number of reasons cited as to why the digital photograph is not – and indeed cannot be – indexical.
Often, the first is that the digital photograph is so easily and so seamlessly manipulated; in the words of William J. Mitchell, the referent has become “unstuck”.  Where Barthes saw the (chemical) photograph as referring to a “necessarily real thing which has been placed before the lens”,  Mitchell argues that there is no such guarantee in the case of a digital image.  However, while it is true that software like Photoshop makes it easier to manipulate or fake a photograph, it has to be acknowledged that photographic manipulation is not a practice confined to digital images. Addressing Mitchell’s writing directly, Lev Manovich argues that Mitchell can only make this particular claim precisely because he identifies manipulated photography as lying outside the mainstream of photographic practice, effectively sweeping a great many examples of non-digital manipulation under the carpet.  Photographic manipulation is as old as photography itself,  with examples ranging from the fantastic (e.g. The Cottingley Fairy photographs ) to the everyday (e.g. touching up ladies’ faces ).
Jay David Bolter and Richard Grusin similarly argue that “a digital photograph can be as transparent as an analog one” and, furthermore, “it is purely a cultural decision to say that darkening the color values of an image digitally is a manipulation but leaving the print longer in the developing bath is not”.  In other words, while the digital photograph has a greater capacity for manipulation that is not to say that every digital photograph has been manipulated – it may well be as ‘transparent’ as an image printed in the darkroom. Mitchell, however, would argue that the very knowledge that the photograph might have been manipulated is enough to shake our faith in its transparency, its straightforward connection to the real.
This first argument also closely ties the index to the ‘documentary value’ of the photograph; to its possessing a visual likeness of its referent and its ‘truthfulness’. Indexicality, however, actually has very little to do with visual likeness and perhaps even less to do with ‘truth’. Indexes often share a visual likeness with their referents, photographs being a case in point, but they do not have to. Consider a weathervane (one of C.S. Peirce’s examples of an index): it does not look like wind but indicates the presence of wind through its movement.  Andre Bazin, quoted by both Roland Barthes and Rosalind Krauss, similarly states that:
No matter how fuzzy, distorted, no matter how lacking in documentary value the image may be, it shares, by virtue of the very process of becoming, the very being of the model of which it is the reproduction. 
The photograph’s status as an index does not preclude it from acting as another type of sign, as such the photograph might be said to share some qualities of an ‘icon’ – at least in terms of its visual likeness.
This brings me to the second argument often levelled against the digital photograph’s indexicality: that it represents not a physical imprint of light but information storage. Rosalind Krauss is especially clear on this point, going as far as to place digital photography in the realm of the iconical (the realm of painting) instead of the indexical.  Her argument, which draws heavily on Friedrich Kittler,  states that the digital photograph has no intrinsic physical link to the referent because the image is not inscribed as constantly varying tones on a light-sensitive material, but stored as mathematical colour values. This fact also factors into the previous argument, as these colour values can be easily altered without any physical damage to the photograph.
The digital photograph is a mathematical model – that point is not in dispute. It is a very precise mathematical model, consisting of a grid of pixels, each one with a colour value expressed in numbers that duplicates as closely as possible the colour of the referent. However, there is no reason that these values have to be output in a form we recognise as a photograph, they could equally be expressed as numbers (204, 0, 153 refers to the pink colour used on this website), as sound, or in any other form we care to give them. Krauss (and other similarly minded writers) see this reduction to numbers as severing the photograph’s link to the referent, as the referent no longer impresses itself on the media the way it does in a chemical photograph.
Peirce too placed mathematical models in the realm of the iconical:
The reasoning of mathematicians will be found to turn chiefly upon the use of likenesses, which are the very hinges of the gates of their science. The utility of likenesses to mathematicians consists in their suggesting, in a very precise way, new aspects of supposed states of things. 
But would Peirce have made an exception for the digital photograph? If the chemical photograph is an index thanks not to its visual likeness but its method of production, can the same not be said of the digital photograph? After all, it too is forced to “correspond point by point to nature”  as best it can, so does it not share something of the chemical photograph’s “process of becoming”? 
Putting aside the technical arguments for the time being, consider this: If the index is the sign essential to camera work – the quality of photography that gives it its particular power – then surely the non-indexical digital photograph should be something of a weak force, but it is not. Digital photographs have power, just as chemical ones do. Mary Anne Doane points to images of suffering, such as those captured during the 9/11 attacks and in Abu Ghraib, as particularly powerful examples that “point to the persistence and strength of an indexical imaginary even in the realm of digital photography.”  This suggests that the index continues to assert its power in spite of all the arguments against its existence.
It is widely argued that the index, defined as the physical imprint of light in chemical photography, does not exist in digital photography. The two main arguments against it are that digital photographs have a far greater capacity for manipulation and that the reduction of an image to colour values severs the physical connection to the referent. However, indexicality actually has very little to do with visual likeness.
 William, J. Mitchell, The Reconfigured Eye: Visual Truth in the Post-Photographic Era (Cambridge, MT: The MIT Press, 1992).
 Roland Barthes, Camera Lucida. (London: Vintage, 2000).
 William J. Mitchell, The Reconfigured Eye.
 Lev Manovich, “The Paradoxes of Digital Photography” [Online], 1994, http://manovich.net/index.php/projects/paradoxes-of-digital-photography (accessed 4 November 2014).
 See Mia Fineman, Faking It: Manipulated Photography before Photoshop (New York: Metropolitan Museum of Art, 2012) for a history of manipulated photographs from the pre-digital era.
 The Cottingley Fairy photographs are a series of photographs taken by two young British girls which purport to prove the existence of fairies, most famously fooling Sherlock Holmes’s creator Sir Arther Conan Doyle. See: Josh Jones, “Arthur Conan Doyle and the Cottingley Fairies: How Two Young Girls fooled Sherlock Holmes’ Creator,” Open Culture, 23 January 2013, http://www.openculture.com/2013/01/arthur_conan_doyle_the_cottingley_fairies_how_two_young_girls_fooled_the_creator_of_sherlock_holmes.html (accessed 4 November 2014).
 Jung Chang notes that touching up ladies’ portraits was common practice in the early twentieth century. Portraits taken of the Chinese Dowager Empress Cixi were touched up (e.g. to remove wrinkles) before being presented to her, so as to avoid causing offence. See: Jung Chang, Empress Dowager Cixi: The Concubine who Launched Modern China (London: Jonathan Cape, 2013).
 Jay David Bolter and Richard Grusin, Remediation (Cambridge, MT: The MIT Press, 2000).
 C.S. Peirce, The Essential Peirce: Selected Philosophical Writings, eds. Nathan Houser and Christian Kloesel (Bloomington: Indiana University Press, 1992).
 Andre Bazin, “The Ontology of the Photographic Image,” Film Quarterly 13(4) 1960: 4 – 9.
 Rosalind Krauss, “Rosalind Krauss on Tacita Dean’s Film” [Podcast] (London: Tate Galleries, 2012), http://www.tate.org.uk/context-comment/audio/rosalind-krauss-on-tacita-deans-film (accessed 4 November 2014).
 Krauss cites Friedrich Kittler, Gramophone, Film, Typewriter (Stanford, CA: Stanford University Press, 1999).
 C.S. Peirce, The Essential Peirce, 6.
 Bazin, “The Ontology of the Photographic Image.”
 Mary Ann Doane, “Introduction,” Differences 18(1) Spring 2007: 5.