The Deutsche Börse Photography Prize, now in its 19th year and with a prize of £30,000, recognises the most significant contribution to photography in Europe, in either exhibition or book format. This year's shortlist includes examples of both, and of both video and stills photography.
'Three of the artists move seamlessly between film and photography in a way that really deepens their level of engagement,' Brett Rogers, the director of the Photographers' Gallery in London, which will host an exhibition of the shortlisted artists, said. 'People might have thought that digital would constrain the medium, but I feel quite the opposite. It's really opening up possibilities for the next generation.'
The nominees are Alberto García-Alix (born 1956, Spain) for his publication Autorretrato/Self-Portrait; Jochen Lempert (born 1958, Germany) for his exhibition Jochen Lempert at Hamburger Kunsthalle; Lorna Simpson (born 1960, USA) for her exhibition Lorna Simpson (Retrospective) at Jeu de Paume, Paris; and Richard Mosse (born 1980, Ireland) for The Enclave, a six-screen video installation at the Venice Biennale.
Mosse began experimenting with Aerochrome film in 2009. Invented for reconnaissance during the Second World War, it registers infrared light normally invisible to the naked eye, and turns things a kitschy pink in the process. Mosse had been working as a documentary photographer in Iraq and Gaza, and had reached a point 'where I was looking to throw away my crutches and take a leap in the dark'. This coincided with Kodak announcing it was discontinuing Aerochrome. 'I would never normally have touched the stuff with a bargepole,' he said. 'But it's a very eccentric medium. I thought it might put me in an uncomfortable place where I didn't know what I was doing, and that's a good place to be as an artist. Originally it had been used to reveal enemy camouflage, so I asked myself, where is an unseen narrative? Where is the most unlikely place you would use this film?'
A few days later he boarded a plane to eastern Congo. He had tested the film only once 'and it looked vaguely OK. Actually it looked pretty bad, but I went anyway.
There was a certain amount of self-destruction about that first journey. I wanted to do something that would probably fail.'
In 2008 the International Rescue Committee estimated the death toll in war-torn Congo at 5.4 million. 'But we don't hear about it, because they're dying from a lack of sovereignty and constant displacement, shitty diseases.' Four rainy seasons a year make the jungle voracious. The architecture is built to be abandoned because the front lines are constantly shifting. News - of massacres and mass rapes - takes days to emerge from the jungle. 'By the time photographers arrive there is nothing left to see. It was this lack of trace that interested me, and ultimately the failure of documentary photography,' Mosse said. 'Conflict is complicated and unresolvable, and it's not always easy to find the concrete subject, the issue, and put it in front of the lens.'
Short of money, he stayed in Catholic missions. He oriented himself by talking to the handful of correspondents left in Kinshasa but the longer he spent there, the more people he spoke to, the more rebels he encountered, the more convoluted his understanding became. There are at least 30 rebel groups in eastern Congo. 'Many of them used to have an ideology but they've long since forgotten it. They fall into alliances with each other, then renounce them.' It took time to find the rebels.
'You'd take a Land Cruiser as far as you could go, about half a day depending on rain, and then you'd walk, stay a night, and then walk another day, until you passed the front line and into the enclave. Once you're in there, time changes, as does logic. Some of these rebels believe they're bulletproof.'
Mosse spent a lot of time by himself. 'I appreciate the retreat into your imagination that happens. I love the blank canvas. I spent hours watching lizards creeping up on clouds of mosquitoes. I brought Conrad's Heart of Darkness with me and I read it over and over and over and over. I fixated on certain paragraphs. Now I can't touch it.'
On his return 'I almost didn't process the film, I was so horrified by my impending bankruptcy. I was looking for jobs as a dishwasher.'
But then he happened to look at a picture he had taken of a landscape. 'I almost ignored it because it was a pretty picture, then I realised what had been staring me in the face the whole time. The pink pushed the viewer into this extraordinary space, way past the threshold of the imagination and into science fiction, something pulsating, nauseous. We don't see in pink, but we don't see in black and white either - whichever way you look at it, documentary photography is a constructed way of seeing the world.'
In 2011 he published the work in a book, to considerable acclaim. About the same time he began to hear rumours of 16mm infrared movie film that still existed in the depths of a freezer in Hollywood. It took him more than a year to track it down, but eventually he found himself back in eastern Congo with the filmmaker Trevor Tweeten. They had 35 reels of film - each reel lasts about 11 minutes - and an old-fashioned Arriflex SR2 movie camera.
When they returned Mosse couldn't find anyone to process the film. 'I went from lab to lab thinking, I'm ruined, I can't do anything with this amazing footage. But at last I found an old-timer in Denver. It took him six months but finally he cracked it.'
The finished piece has a mesmeric quality. Tweeten perfected a floating gaze, and with its forest of screens, one feels almost lost. Figures pose and strut for the camera, sometimes they dance. One moment it's you and a wide expanse of bush, susurrating quietly, the next, a young girl is singing in one ear while gunfire erupts in another. It's physically immersive and devastatingly beautiful but all the time you are moving forwards towards the horror at its heart.
Mosse has been back to Congo once since. 'I went to get closure, to say goodbye. I'd hired a car, and the driver was so arrogant, he drove off a bridge. It flipped, fell about 25 feet. We all survived but I had to perform first aid on the driver in the middle of a cloud of mosquitoes, in the middle of swamp, in the middle of night, in the middle of nowhere with all my cameras everywhere, and I thought this is it, I'm finished. I'm finished with Congo.'
The Deutsche Börse Photography Prize exhibition, the Photographers' Gallery, London W1, April 11 to June 22.
The Vinyl Factory and Edel Assanti present The Enclave at the Vinyl Factory Space, Brewer Street Car Park, London W1, April 4-26