The German painter's long-time embrace of photography has resulted in an exhibition that shows how bleakly eloquent a snapshot can be, writes Brian Dillon

In 1927, in an essay entitled 'Photography', the German writer Siegfried Kracauer reflected on the nature of a technology that seemed to him to have usurped, over the course of a century, the place of human memory.

Photography, he declared, lacks the intimate embrace of its object that memory affords us: it records merely a brute facticity, cut off from the memorial "monogram" inscribed by proper recollection. Memory, wrote Kracauer, bathes its object in a kind of aura, an emotional haze that the photograph, with its blank insistence on a single instant, is incapable of conjuring.

And yet the author was clearly bewitched by certain photographs, describing in loving and entranced detail a photograph of his grandmother taken 60 years earlier and a contemporary publicity still of a young actress. Both women, despite their utterly different attire and attitudes, have something spectral about them, an air of melancholy that led Kracauer to conclude that a photograph is a sort of historical phantom, an eerie revenant hovering somewhere between beauty and terror: "Now the image wanders ghost-like through the present, like the lady of the haunted castle. Spooky apparitions occur only in places where a terrible deed has been committed."

If any contemporary artist can be said to have explored fully that odd space between photography's twin faculties - its capacity to record and resonate at the same time - it is surely Gerhard Richter. His Atlas, on show at the Whitechapel gallery in London, is a vast and varied archive of the artist's engagement with the photographic image in all its disparate, haunting and delightful forms.

Best known (and sometimes disparaged) as a painter whose most famous works are painterly renderings of photographic originals, Richter has, over the past four decades, amassed a photographic collection that has become a work of art in itself: a meditation on the photograph as an index of very individual obsessions and grandly historical movements.

At the same time as it memorialises Richter's working methods - many of the images collected in Atlas are the models for his sometimes modestly sized, often monumental paintings - it is also a kind of museum, a cabinet of cultural curiosities drawn from the 20th century's ambiguous romance with the snapshot.

Born in Dresden in 1932, Richter spent the early years of his artistic career designing propaganda banners and training as a mural painter under the communist regime. In 1961, shortly before the construction of the Berlin Wall, he moved to Düsseldorf to continue his studies; he found himself in a uniquely German mid-century artistic milieu, a moment at which progressive German artists seemed to be rejecting wholesale the aesthetic options then available.

Leaving behind the strictures of Soviet socialist realism and the exhausted pre-war heritage of the European post-cubist avant-garde, Richter was equally unconvinced by the painterly resurgence promised by the abstract expressionists of the New York school.

Indeed, painting itself seems to have been the problem for Richter. Around this time he painted his first photograph-based picture, derived from an image of Brigitte Bardot. He later wrote: "I had had enough of bloody painting, and painting from a photograph seemed to me the most moronic and inartistic thing that anyone could do." The method stuck, and its ostensible idiocy became instead the starting point for an astonishingly complex and sensitive exploration of the relationship between painting and photography.

In 1964 Richter began to collect photographs to transform into paintings, later freeing them from their immediately practical and provisional occasions: arranging them on panels that would form the basis of his Atlas. First exhibited in 1972, Atlas comprised 315 panels: a figure that has risen, in the current incarnation of Atlas, to 691, made up of more than 5,000 individual images. Arranged in simple grids of varying sizes, the pictures present an extraordinary anatomy of the photograph in the last century, ranging from private family snapshots through advertisements culled from magazines to the photographs of the Holocaust that make up (for Richter and the viewer alike) the collection's most troubling section.

Richter is fascinated by the suggestive possibilities of the arrangement of his images into gridlike patterns. Among the first panels encountered by a visitor to the current exhibition are numerous presentations of clouds and seascapes, ravishing (if also banal: indistinguishable from our own touristic snapshots) depictions of a purely romantic anachronism: the glory of a sunset or tormented cloudscape.

Replicated to minutely differing infinity by their serial arrangement, these photographs are at once gorgeous and debilitated, their sublimity crippled by repetition.

Some are also distinctly uncanny in their strange mismatching of sea and sky; on closer inspection, air and ocean turn out to have been sutured together from different photographs (a common practice in Victorian photography, before the technology could capture drifting cloud and roiling wave with equal intensity). Romantic notions of nature, these panels suggest, are the effect of mere technical hubris. Richter frames several of the grand vistas with drawings of impossibly vast interiors: fantasised gallery spaces in which tiny human specks survey natural panoramas that they have, after all, actually invented.

The effect of Richter's perpendicular presentation of such photographs is to set up a curious tension between the photograph as testament to a distinct reality and its capacity to transform that vivid actuality into an endlessly replicable flatness: to turn lived history into frozen taxonomy.

Where a comparable artist such as August Sander was interested, in his huge photographic record People Of The Twentieth Century, in capturing the amazing disparity of human forms, Richter uses his found images to oddly uniform effect. His 48 portraits of famous men (and only men; women, he suggests, would have introduced an unruly variety of dress and coiffure) are bizarrely alike in their suited and sober regularity. Rilke, Einstein, Mahler and Stravinsky all turn their heads to the same angle; only Kafka stares from the centre of the grid with a look that suggests he won't be tamed by the frame.

Similarly, aerial photographs of cities (and of models of planned cities) are at once reduced to abstraction and suffused with memory and historical mourning. An occasional image has been blurred to the point where it looks ruined: a fogged reminder of the artist's obliterated birthplace.

Richter has a habit of making contradictory comments about his slowly developing photographic collection. For every pronouncement that admits to a Warholian fascination with the banal and the uniform ("I blur things to make everything equally important and equally unimportant"), he has made another that claims for the photograph an irreducible intimacy with the historical horrors of his lifetime. If his work has been linked both to the affectlessness of pop art and to the media-entranced sheen of postmodernism, Richter likes to disrupt such readings with a disreputable emotionalism, mischievously announcing to one slightly appalled critic that his was in fact an art of old-fashioned longing, "for lost qualities, for a better world - for the opposite of misery and hopelessness".

What is certain is that he has sometimes attempted to fit the most recalcitrant historical material into his otherwise seamlessly encyclopedic Atlas. Most notoriously, he has exhibited blurred photographs and photograph- derived paintings of the members of the Red Army Faction, the so-called Baader-Meinhof Group, whose supposed suicides in custody in 1977 constituted such a fearful rupture in post-war Germany's sense of itself.

More shocking still, in the distanced context of the collection, are the photographs of the Holocaust with which Richter struggled at one point, trying (and failing) to make them "paintable".

Among the blurred and, alarmingly, hand-coloured photographs of the death camps and other Nazi atrocities, one photograph, repeated three times by Richter, is especially arresting. It shows a young man and woman being hanged by German soldiers (the woman, in fact, is the adolescent Belorussian Jewish partisan Masha Bruskina, executed in 1941).

The three versions of this photograph are dispersed across adjacent panels. In order, perhaps, to fit each one to its grid, Richter has progressively reduced the distance between the two teenagers, excising the centre of the image so that in the final, tiny photograph they are as if about to touch. At the Whitechapel show you have to crouch on the floor at the bottom of the panel to discern between them the faintest incision on the surface of the print.

There is something quite unbearable about this manufactured proximity: it reminds us of the impossibility of traversing the historical gulf between ourselves and this photographed horror but implies too that a photographic memory may be something more than a mere ghost. Richter's prodigious Atlas, with its expanding constellation of lost and found memories, suggests that sometimes a snapshot is more bleakly eloquent than the most laborious painterly imaginings. As Richter puts it: "I never wanted to accuse anything, except life maybe, and how shitty it is."

Gerhard Richter: Atlas is at Whitechapel gallery, London, until March 14th