Deutsche Börse Prize, Photographers' Gallery, review Coming from Japan, South Africa, America and Britain, the four excellent artists this year nominated for the Deutsche Börse Photography Prize are as far apart in their work as they are on the global map. Two of the nominees are not photographers, but make art about photography. At a time when our photographic heritage is threatened by new technologies, they highlight how the medium still occupies a unique place in our cultural canon. One of them is English artist John Stezaker, who has been making photographic collages since the early Seventies. His edits of old postcards, portraits and film stills are deceptively simple. The faces of a man and a woman meet, not seamlessly, but with enough of a proportional match - and character mismatch - to send the mind into overdrive. When the Whitechapel Gallery ingeniously staged a retrospective of his work last year, these Dada tributes seemed fresh and contemporary. The charm of Stezaker's material is its datedness, and the fact that each image was constructed for a use which escapes us today. His experiments with the images resound particularly in the age of Photoshop, a world suspicious of the politically or culturally sensitive tweaks that regularly catch us out. Working in a similarly theoretical vein is Christopher Williams, a difficult artist but an interesting one. His highly-finished images, made by commercial photographers whom he employs, take bafflingly different forms but are always in dialogue with one another. He has just three works here, inviting the question: what do a dumpy grey hay bale in a dark field, a finger clicking the "flash" button on a photographer's light box, and a developer's dark room have in common? Answer: each motif reinforces the photographer's role as giver and taker of light. Just as Monet steeped the golden hay bale in romantic mysticism, so can the photographer suck it out - with a click. Rinko Kawauchi's images of stickily suggestive forms and textures - a white spider on white wall, fish scales and raw meat - slip easily in and out of your subconscious, so that you are left with just a dreamlike impression of what you've seen. Her latest series, "Illuminance," is accompanied by a wonderful film featuring the same images. Why both? The richness of details in the moving image, such as a soft breeze on a stray bubble or the amplified gargle of a goldfish, can only enhance Kawauchi's strange paradise, and certainly makes the accompanying pictures paler by comparison. On an adjacent wall, Pieter Hugo's monumental portraits of Ghanaian workers at a dump site for old technical equipment reinforce your faith in the still photograph. Their stillness is everything. Like the portraits of Nigerians and hyenas for which Hugo is famed, the men and women who stare intensely back at us here - droplets of perspiration forming at their pores - are snoozing volcanoes. Discarded keyboards visible in the background remind us how quickly information and materials are chewed up in the West, but how little has changed in recent years on the other side of the world's economic belt. No other artist working today encapsulates the sad complexities of Africa with such subtle perfection. For that reason Hugo may take the £30,000 prize money in September, but I hope Stezaker gets it. From July 13 until Sept 9