Any discussion on the index, no matter the context, must at some point refer to the writings of C.S. Peirce who, as I noted in my first post on the index, outlined three principle types of sign: likeness (icon), indication (index), and symbol. To recapitulate: an icon is a sign that conveys an idea of the thing it represents simply by imitating it (as in a drawing), an index is a sign that shows something about its referent on account of being physically connected to it (as in a footprint), and a symbol is a sign that has come to be associated with its meaning by usage (as in words). 
At first, it might seem that the photograph should fit most comfortably into the category of the icon, on account of its visual similarity to its referent, however, Peirce tells us that this is not the case. The photograph, according to Peirce, is an index by virtue of its method of production, a method that forces it to correspond “point by point to nature”.  This is reinforced by Andre Bazin’s assertion that the photograph comes to be indexical through the “process of its becoming”, as opposed to any likeness or documentary value. 
Placing the photograph into the category of the index does not, however, mean that it cannot also act as an icon, or even a symbol.  Indeed, while Peirce’s original proclamation of the photograph as an index is widely known and often quoted, it is less often acknowledged that Peirce also ascribed iconical qualities to the photograph.  The ‘dicisign’, of which a photograph is an example, is a secondary type of sign which comprises both index and icon; and, unlike a pure index, conveys information. 
A ‘pure index’ is essentially an empty sign, it cannot convey information on its own – only point to or indicate something. Although Peirce may initially have placed the photograph into the category of the index by virtue of its process of becoming, if we consider how it draws our attention to something in the world we begin to see it also acts as a ‘pointer’. Roland Barthes described this very quality in Camera Lucida, when he observed that people often look through a photograph instead of at it.  In other words, when we say to someone ‘look at this’ and show them a photograph, we often do not mean the printed (or screenic) image, but what it shows us: its referent. In this context, the photograph is acting not as an embalmed memory but as a means to single out something for our attention. However, the photograph could not do this if it were a pure index, because the index cannot tell us what ‘this’ is. It is the photograph’s iconical element that tells us what ‘this’ is.
Consider again the photograph of the Siamese cat – how do we know it is a Siamese cat? Unlike the computer, which relies on metadata, we know it is a photograph of a Siamese cat because it looks like a Siamese cat: this is the photograph’s iconical dimension. Its indexical dimension stems from the fact that it has been, by method of production, forced to correspond to one particular Siamese cat. It points to ‘this’ Siamese cat.
The act of pointing lends the photograph an indeixcality that is distinct from the trace or footprint (which as I already noted has often been the fixation of photographic theory ): one that is ‘performative’.  This ‘performative’ index is still physically connected with the referent, but it is through actions as opposed to chemistry; it might be proposed therefore that it is the act of pointing, as opposed to the physical trace, which allows the digital photograph to speak with such power. The digital photograph’s “imaginary indexicality”  may not be a figment of the imagination.
David Green and Joanna Lowry take this a step further when they suggest that for many contemporary art practitioners, the importance of the photograph is not so much as a record of the real world (documentary), but the act of photographing itself, or rather what it is that the photograph denotes.  Think of it like a selfie taken at the top of Ben Nevis, the point of the photograph is not so much to show people the top of the Munro (though it might) but to testify that the photographer reached its peak. Many works of contemporary photography can be thought of as declarations: of the presence of the photographer or more simply the real world itself. 
If we can understand the (chemical) photograph as both icon and index, we can also see that the distinction between icon and index is not as black and white as it might first appear. Why is it then that when Rosalind Krauss describes the digital photograph as “iconical” it implies a profound change from its chemical forebear?  The difference is that Krauss’s argument hinges on the index being an imprint, not a pointer. In this sense, her initial point still stands: the digital photograph is not a physical imprint of light. This point is not in dispute, the digital photograph is information; and while one may attempt to establish a physical link in the flow of electrons  it cannot be denied that the digital photograph is composed of colour values.
The digital photograph, or any other digital method of recording for that matter, has a resolution. It can be argued that it to is forced by method of production to correspond point by point to nature, but only to the degree that nature can be registered as a mathematical colour value. When light falls on the digital sensor it does not ‘expose’ the photosensitive cells, rather the cells measure the light and record the closest colour that can be represented by the RGB colour model.  A colour that falls between two thresholds will be rounded up or down to the nearest value. These colour values can be represented numerically, for example (255, 0, 0) is the colour ‘red’, (0, 255, 0) ‘green’ and so on.
Much in the way that words are associated with objects, these colour values are associated with colours. The computer will be able to reproduce the colour based on these values; likewise, someone familiar with the notation system – a programmer for example – will be able to visualise the colour to some degree. The digital photograph, therefore, is also symbolic.
The digital photograph then displays characteristics of at least two of Peirce’s principle types of sign: icon and symbol. Its iconicity is not new; and although a degree of interpretation enters into the process this doesn’t necessarily equate to a complete loss of indexicality. The colour values may be an interpretation but we need to remember that any printed chemical photograph also involves a degree of interpretation – only in this case it is the printer exposing the paper in the darkroom who adjusts the colour of the print to make it fit with the scene captured.  Furthermore, if it is accepted that the index does not have to be a physical imprint, it can be argued that also has an indexical dimension that is performative.
 C.S. Peirce, “What is a Sign?” in The Essential Peirce: Selected Philosophical Writings, eds. Nathan Houser and Christian Kloesel (Bloomington: Indiana University Press, 1992), 5.
 Ibid., 6.
 Andre Bazin, “The Ontology of the Photographic Image,” Film Quarterly 13(4) Summer 1960: 8.
 C.S. Peirce, “What is a Sign?”.
 Braxton Soderman, “The Index and the Algorithm,” Differences 18(1) 2007: 153 – 186.
 Roland Barthes, Camera Lucida (London: Vintage, 2000).
 Catherine M. Weir, “The Index” [Online], http://www.cmweir.com/the-index/ (accessed 9 December 2014).
 David Green and Joanna Lowry, “From Presence to the Performative: Re-thinking Photographic Indexicality” in Where is the Photograph?, ed. David Green (Brighton and Maidstone, UK: Photoforum and Photoworks, 2003), 47 – 60.
 Mary Ann Doane, “Introduction,” Differences 18(1), Spring 2007: 5.
 David Green and Joanna Lowry, “From Presence to the Performative.”
 Rosalind Krauss, Rosalind Krauss on Tacita Dean’s FILM [Podcast], Tate, 12 March 2012, http://www.tate.org.uk/context-comment/audio/rosalind-krauss-on-tacita-deans-film (accessed 4 February 2013).
 Laura Marks argues that electronic images can be considered indexical based on the interaction of electrons, but digital images cannot, because the chain is broken when the image is converted to information. See: Laura Marks, “How Electrons Remember” [Online], Millenium Film Journal, 34, Fall 1999, http://www.mfj-online.org/journalPages/MFJ34/LMarks.html (accessed 10 December 2014).
 Aden Evens, “Concerning the Digital,” Differences 14 (2) 2003: 49 – 77.
 Daniel Rubinstein and Katrina Sluis, “The Digital Image in Photographic Culture” in The Photographic Image in Digital Culture (2nd Edition), ed. Martin Lister (Abingdon: Routledge, 2013), 22 – 40.