Nicholas Nixon's 40 years photographing the Brown sisters The second (and first surviving) portrait from 1975: left to right, Heather, Mimi, BeBe and Laurie, in New Canaan, Connecticut. Photograph: © 2014 Nicholas Nixon "The series grew out of boredom," Nicholas Nixon said recently of his ongoing long-term portrait project, The Brown Sisters, in which he has photographed his wife, BeBe, and her three sisters every year over a 40-year period. We'd go down to visit BeBe's parents on weekends. It was kind of boring, a lot of socialising, we were expected to show up for dinner every day ... Out of a friendly desperation, I said: "Let's take a picture."" Nixon met BeBe in 1970, when he was 21 and she was 20. They were married the following year. In August 1974, he took his first group portrait of the sisters, but discarded it as intrinsically uninteresting. The first portrait that appears in his book The Brown Sisters, which is now in its third - expanded - edition, is from 1975, and it shows four fresh-faced young women who each stare at the camera with differing degrees of acceptance. Four years in to the series - the 1978 portrait. Photograph: Nicholas Nixon The following year, after what he tactfully describes as "some degree of negotiation" with his subjects, two of whom wanted to be in the middle of the portraits some of the time, Nixon settled on the order that would define the series: left to right, Heather, Mimi, BeBe and Laurie. He also chose to use an 8x10in view camera on a tripod and black-and-white film. It has been that way ever since. The series, which showed in its entirety for the first time at the Fraenkel Gallery booth at Paris Photo last week, opens at MoMA, New York this week. It is, among other things, a testament to the power of one great, simple idea. Out of that idea, though, Nixon has made a series with an extraordinary cumulative power that rests on photography's singular ability to capture the passage of time and, with it, human ageing and the lurking shadow of mortality. "We are all aware of time passing and us not being aware of it while it's passing," he has said. So seeing the sisters, for a lot of people, gives them a reliable marker that a year has passed. When he took the first portrait, Mimi was 15, Laurie 21, Heather 23, and BeBe 25. The 10-year difference between the youngest and oldest is not so dramatic here, but begins to register in the following years. By 1981, for instance, Mimi still looks like a teenager, though she is 21, while BeBe, sitting slightly behind the other three, looks older and paler, as if she has experienced much more. The 1988 portrait of the Brown sisters. That dynamic shifts again in the following year's portrait, in which BeBe looks softer and more at ease, her looks echoing those of Heather, while Laurie and Mimi seem both more defiant and inquisitive before Nixon's camera. The sisters, interestingly, are the arbiters of each image, with all four having to agree on the choice of photograph every year. As with similar projects - Steve Pyke's more obsessive chronicling of his sons Jack and Duncan's passage from childhood into adulthood, for instance - there are series within the bigger series when time seems to move in surges, so much so that you wonder what may have happened in the intervening year to render an individual face so much older- or sadder-looking. This, too, is one of the powerful undercurrents of The Brown Sisters - the sense that 12 months is just a mathematical marker, and that it is life, rather than time, that makes us all age to different degrees. Sisterly bonds are strong in the 1999 portrait. But there is - as always with photography - something else going on here, to do with the artist's eye for light, shade and composition as well as the camera's often unforgiving gaze. In Nixon's portrait from 1996, his shadow (and that of his plate camera) falls over the sisters as if alerting us to his invisible and governing presence. Mimi, as in the very first portrait, still looks defiantly at him as if facing down his camera. In an earlier portrait from 1986, he has moved in close, squeezing the four into the frame. This is perhaps the most tender and intimate portrait of sisterhood in the series, though as they grow older, the bonds between them seem to deepen and their individual expressions soften into another kind of acceptance, more giving than stoical. This year's portrait. There are other intriguing subtexts to The Brown Sisters: namely that Nixon is an only child engaging with a closely bonded set of female siblings, an outsider not just because of his camera and the detachment it confers. Ultimately, though, The Brown Sisters fascinates us because it is a rigorously formal, beautifully executed version of the family photograph - albeit one that hangs on gallery walls rather than a photo album. In these four strangers, we see some semblance of ourselves and our inexorably unfolding lives. Hanging over the whole project is the big what-if of mortality. What if one of the sisters dies? Will Nixon carry on? He is a photographer, with all the hard-nosed obsessiveness that entails, so the answer is almost a foregone conclusion. "We joke about it," he said recently. But everybody knows that certainly my intention would be that we would go on forever, no matter what. To just take three, and then two, and then one. The joke question is: what happens if I go in the middle. I think we'll figure that out when the time comes.