Indexicality is perhaps the most unfamiliar concept.
Peirce offers various criteria for what constitutes an index.
An index 'indicates' something: for example, 'a sundial or clock indicates the time of day' (Peirce 1931-58, 2.285).
He refers to a 'genuine relation' between the 'sign' and the object which does not depend purely on 'the interpreting mind' (ibid., 2.92, 298). The object is 'necessarily existent' (ibid., 2.310). The index is connected to its object 'as a matter of fact' (ibid., 4.447). There is 'a real connection' (ibid., 5.75). There may be a 'direct physical connection' (ibid., 1.372, 2.281, 2.299). An indexical sign is like 'a fragment torn away from the object' (ibid., 2.231).
Unlike an icon (the object of which may be fictional) an index stands 'unequivocally for this or that existing thing' (ibid., 4.531).
The relationship is not based on 'mere resemblance' (ibid.): 'indices... have no significant resemblance to their objects' (ibid., 2.306).
'Anything which focusses the attention is an index. Anything which startles us is an index' (ibid., 2.285; see also 3.434).
Peirce noted that a photograph is not only iconic but also indexical: 'photographs, especially instantaneous photographs, are very instructive, because we know that in certain respects they are exactly like the objects they represent.
... Since the photographic image is an index of the effect of light on photographic emulsion, all unedited photographic and filmic images are indexical.
However, digital imaging techniques are increasingly eroding the indexicality of photographic images.
Documentary film and location footage in television news programmes depend upon the indexical nature of the sign. In such genres indexicality seems to warrant the status of the material as evidence.
Photographic and filmic images may also be symbolic: in an empirical study of television news, Davis and Walton found that a relatively small proportion of the total number of shots is iconic or directly representative of the people, places and events which are subjects of the news text. A far greater proportion of shots has an oblique relationship to the text; they 'stand for' the subject matter indexically or symbolically (Davis & Walton 1983b, 45).