There are a number of pictures in William Eggleston’s Guide that include dogs happily wandering the dusty roads of Louisiana and Mississippi.

The expressive characters of these dogs fit perfectly with the chance encounters of the people that have become iconic examples of Eggleston’s oeuvre. I have always thought of those dogs as self-portraits, and having seen Eggleston’s method of working in Michael Almereyda’s 2005 film William Eggleston in the Real World, I am convinced of it.

William Eggleston is the defining artist of contemporary photography. The Memphis-born septuagenarian's place in the pantheon of masters is as deservedly concrete as Kubrick’s in cinema, Beckett’s in theatre and Bacon’s in painting. So for the National Portrait Gallery in London to put on the exhibition William Eggleston Portraits, they were always going to be hitting a winner, and it is with a breath of relief that they have done so with appropriate tact and grace.

2002 was the last time a major show of Eggleston’s was exhibited in London, but with Almereyda’s film, BBC’s documentaryThe Colourful Mr. Eggleston (2009), and major publications like Steidl’s Chromes (2011) for instance, his pictures have continued to thrive in an increasingly noisy and cynical world. For Eggleston has never stopped working, and since his groundbreaking MoMA show in 1976, the appreciation and hunger for his work has never subsided. Even most recently, in a New York Times magazine feature that includes portraits and a short video piece by the acclaimed German photographer Wolfgang Tillmans, Eggleston reveals that he still takes pictures every day.

I was an art student in my hometown of Dublin, Ireland, in 2002, and it was not a tutor, but a painter friend, who informed me of this great artist who possessed a cutting wit in equal abundance to his blisteringly beautiful use of light and colour. When I marched to the university library to find the only book of Eggleston’s there – it was Guide, originally published to coincide with the MoMA show and probably the most important book an aspiring photographer could own – it was like a window smashing and a gust of wind blowing through my maladroit mind.

There are several portraits from Guide on display in the National Portrait Gallery show, but there is one giant that dominates: the cinematic masterpiece from 1969-70, depicting Eggleston’s uncle, Ayden Schuyler Senior, shoulders back and hands in pockets, while standing behind is his assistant Jasper Staples, shoulders back and hands in pockets. This pair in contrasting black-and-white composition amongst the slender tress and fallen leaves in Cassidy Bayou, Sumner, Mississippi, have been written about extensively. It is laden with tales of racial and familial history, though nothing is ever what it seems. The simplicity of this image is equalled by the complexity of its reality and implications. And in person, it is demanding and authoritative, and of course just as relevant in today’s Black Lives Matter/Donald Trump America as it was during Kennedy/Nixon era.

Eggleston has always held a bohemian heart and his many friends, family members and consorts are all present here to bear witness to his effortless investigations. There are early monotone works, created in the 1960s and sometimes using spy cameras with police surveillance film such with the portrait of his mother at home. He processed them himself in his own darkroom. These previously unseen relics, and others like them, evidence a side to Eggleston’s serious self-education in the science of photography. His subjects are usually caught off guard (or totally unaware), in oddly framed snapshots that seem to act as sketches for what would come later. And what does come later are magnificently composed portraits in colour of the very same or similar characters drinking in clubs and bars; their patterned clothing, shimmering hair and soft skin frozen against an almost absent background of darkness. In these pictures we see what would come decades later through the 1990s rave scene portraits that descendants like Tillmans and Juergen Teller would become known for.

So on the one hand there is Eggleston, the alchemist who experiments, physically moving the image during the dye-transfer process for which he is renowned, he dismantles and rebuilds the medium. On the other there is the artist, the life-long practitioner who uses that knowledge and wisdom to create aesthetic beauty and conceptual depth. In both his personal and professional life he mixes pleasure with pain, serendipity with charisma, pouring it back into the work. It is impossible to look at the 1974 work of Karen Chatham, with the artist's cousin Lesa Aldridge lying on a floral couch, without being reminded of Renaissance daughters or Pre-Raphaelite lovers. What of the richly toned self-portrait that defiantly shouts Rembrandt. Indeed, one cannot stand before the sumptuously sun-laden portrait of Marcia Hare lying on her back in Memphis without thinking of some unknown Goddess of Photography. Her eyes closed in contemplation of times to come, her amber hair reflecting the pure American light, her outstretched arms beckoning all to her comfort, while she holds a Kodak Brownie camera pointing back towards her father the Sun.

With remarkable intimacy and invention, this exhibition contains everything about Eggleston’s photographic relationship with the people he has known and loved. Having such an exhaustive quantity of breathtaking material to glean from, it has been curated so incredibly tightly that from the moment one enters to the moment one leaves there is not a feint element to behold. There is no article, review or essay anyone could now write that would ever do justice to Eggleston’s output. One simply has to visit a show like this to grasp how influential and powerful his life’s work has been on so many other artists, from Hollywood to the suburbs of Dublin. Quite simply, it is as impossible to argue with the genius of Eggleston as it is to argue with the dogs that roam the dusty roads of Mississippi.

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William Eggleston Portraits at the National Portrait Gallery, London, until 23 October 2016