The expression «optical unconscious» derives from the acknowledgement by German philosopher Walter Benjamin of the absolutely original contribution of photography and cinema towards the enrichment of human perception. These new technical images helped discover hitherto unknown - i.e. unacknowledged and analysed by perception and therefore restricted to the space of the unconscious or, as he called it, of an «optical unconscious» - movements and dimensions of reality.
Walter Benjamin’s interest for psychoanalysis and its terminology gave rise to his expression, taken from his 1936 essay «The work of art at the age of its technical reproducibility», which has become so in the past few years: «It is through the camera that we first discover the optical unconscious, just as we discover the instinctual unconscious through psychoanalysis». Benjamin recognises photography and cinema have the ability to record aspects of reality that do not fit into the natural optics, namely because they are too quick, small or disperse. The retina frequently receives these aspects; however, they are not transformed into information by the perceptive system. As he demonstrates, these technical images allow a better analysis of the performance of actors in a film, as it becomes «more easily isolated in its constituting elements». On the other hand, close-up and slow motion in cinema, and blow-up and the retarder in photography are not merely means to display known elements from reality, but mainly means to «bring to light entirely new structures of matter».
The clash of the human and the new language brought by the camera, which is so different from that of the human eye, is at stake: «[a] space informed by human consciousness gives way to a space informed by the unconscious. Whereas it is a commonplace that, for example, we have some idea what is involved in the act of walking (if only in general terms), we have no idea at all what happens during the split second when a person actually takes a step. We are familiar with the movement of picking up a cigarette lighter or a spoon, but know almost nothing of what really goes on between hand and metal (...)».
This expression thus conveys an absolutely new feature of technical images: the fact that they may include information that was not retained, processed or even intended by the operator. These images’ previously unseen feature ultimately reserves the ability to surprise the look – to surprise it with the unexpected. This factor did not go unnoticed in the early diagnoses of photography, namely by François Arago, even though the name did not appear then. Arago acknowledged, in his «Rapport»: «When observers apply a new instrument to the study of nature, what they had hoped for is always but little compared with the successions of discoveries of which the instrument becomes the source – in such matters it is on the unexpected that one can especially count.»
Benjamin resumed this concept in his «Little history of photography»: «He first learns of this optical unconscious through photography, just as he learns of the intellectual unconscious through psychoanalysis. Details of structures, cellular tissue, with which technology and medicine are normally concerned – all this is, in its origins, more closely related to the camera than is the emotionally evocative landscape or the soulful portrait. Yet, at the same time, photography reveals in this material physiognomic aspects, image worlds, which dwell in the smallest things, – meaningful yet covert enough to find a hiding place in waking dreams, but which, enlarged and available for formulation, make the difference between technique and magic visible as a thoroughly historical variable» .
 Walter Benjamin, «The work of art at the age of its technical reproducibility (Second Version)» (1955), in Michael W. Jennings, and others (ed.), The Work of Art at the Age of its Mechanical Reproducibility and Other Writings on Media. Translated by Edmund Jephcott, and others. Cambridge, Massachusetts, London, England: The Belknap Press of Harvard University Press, 2008: 37.
 Walter Benjamin, op. cit.: 37.
 Francois Arago, «Rapport sur le daguerréotype», in AA.VV., Du Bon Usage de la Photographie: une anthologie de textes, Centre National de la Photographie, Paris, 1987: 14.
 Walter Benjamin, «Little history of photography», in Michael W. Jennings, and others (ed.), The Work of Art at the Age of its Mechanical Reproducibility and Other Writings on Media. Translated by Edmund Jephcott, and others. Cambridge, Massachusetts, London, England: The Belknap Press of Harvard University Press, 2008: 278-9.