For three decades now, the artist and playwright John Byrne has been sitting regularly for photographer David Eustace, the Glasgow-born photographer who left school at 16 and joined first the navy and then the prison service before settling on a career behind a camera.

The relationship started when Eustace was still a student at Edinburgh’s Napier Technical College, now Napier University. Today, he’s one of the country’s foremost fashion photographers. He has worked for publications such as Vogue, GQ and Tatler and photographed such stellar subjects as Sophia Loren, Sir Paul McCartney, Sir John Hurt and legendary Magnum photographer Eve Arnold. But in a career which also encompasses images of landscapes, Glasgow buskers and clowns, Eustace’s working relationship with Byrne and his abiding friendship with him has resulted in a body of work which is amongst the photographer’s most profound and, you sense, most personal. Byrne’s is certainly the human face Eustace has photographed the most, which must stand for something.

To mark that 30 year anniversary, Eustace has selected just 12 images from the many he has shot over the years and produced a limited edition, hand-crafted boxset. Meanwhile the images themselves are currently on display at the Scottish Gallery in Edinburgh. The exhibition is titled Dear John, A Thirty Year Portrait, which says it all really.

“One of the briefs in my final year at Napier was to make a series of self-portraits,” Eustace explains. “You could photograph anybody but I wanted to make it more interesting. John had just made Tutti Frutti and he was working on Your Cheatin’ Heart at the BBC so I went through them to see if he was up for it. So that was the first time I met him, 30 years ago. That image features in the exhibition.”

That first portrait, shot during a break in production at the BBC’s old headquarters in Queen Margaret Drive, Glasgow, shows a slightly amused-looking Byrne, then 50, smoking a cigarette and staring straight into the camera. Eustace was nervous. Byrne put him at ease by telling him to take his time and calling him Davie. Another black and white image from the same shoot shows Byrne looking almost coquettish, his hand disappearing into his luxurious moustache. A later image has Byrne staring at his feet and wearing a splendid tweed suit in mustard while a stark, three-quarter profile head shot in black and white recalls Jane Brown’s famous portrait of Samuel Beckett. All those images are included in Dear John.

“I’ve only picked three or four sessions and one or two images from each session, so even though it’s my favourite shots, they’re my favourite shots that work together from memories rather than the most interesting shots,” says Eustace, explaining the rationale behind the selection process. “It’s not like I’ve done one five years apart or ten years apart. To put it in perspective, I’ve photographed John three times in the last month but the most recent image in the portfolio was made last year.”

Does he have a favourite image?

“No, because it’s like a jigsaw. One only exists because of the others,” he says. “[The portfolio] is a comment, a glimpse into a bigger event. As much as I do like the portraits selected, they’re as much about things going on in the background in my mind when I was doing them.”

In the most recent shot, a colour image, Byrne sits low in the frame, a trilby hat on his head and his right hand up to his chin. The amused look is absent, replaced by something else. Trust, perhaps, or a sense of calm. It was shot in Eustace’s home in Edinburgh. Another recent image, shot in the garden of Byrne’s home near the capital, shows the subject wearing an overcoat and holding a different trilby, a checked one this time. He's holding it over his face which is entirely obscured by it. Besides the hat only Byrne's hand is visible.

“He lost that hat,” Eustace muses.

I ask him where.

“Fuck, if he knew that he’d find it,” he laughs.

Born in Paisley in 1940, John Byrne turned 80 in January so in many ways Dear John is also a birthday present and a tribute. Although best known for his Slab Boys trilogy of plays and the seminal TV dramas Tutti Frutti and Your Cheatin’ Heart, which featured his future partner Tilda Swinton and handed early breaks to Emma Thompson and Robbie Coltrane, Byrne is first and foremost an artist and illustrator. It’s that which drew Eustace to him.

“I think when I look at John, to me he looks just how an artist should look in my mind. If I had never met an artist and somebody said 'Right pick one of these people' I’d pick him. I think he’s got such a distinctive character. He tries but he doesn’t try too hard. John’s style comes from within. He’s very aware of his style but where a lot of people try to conceive looks, he does it naturally.”

However for Eustace the appeal of Byrne as a subject is about more than his looks and his sartorial elan. Clothes may maketh the man but where Byrne is concerned there’s more to it than that.

“He just oozes character,” is how Eustace phrases it. “What John puts out there is an energy. He just has charisma. He personifies character, for me. I would just describe John as a bundle of creativity. I’ve sat many times watching John painting and he doesn’t necessarily know what he’s going to paint, but he’s got a rough idea and it just comes. It’s like his style, it just comes effortlessly.”

But as much as anything, Dear John is exactly the thing its title suggests: a message from one friend to another. It’s a point Eustace makes in the artist’s statement which accompanies the exhibition and the portfolio. It starts “Dear John” and ends “Your friend, David” and in it Eustace writes: “I revisit these images often and can’t even recall myself when some of them were made. It’s not important. They form cherished memories and moments combined within a frame that offer an insight of a bigger picture … When you answered that initial call all those years ago, I explained my hopes were to make a portrait of you. Thank you, dear John, for saying yes.”

David Eustace: Dear John, A Thirty Year Portrait is at The Scottish Gallery, Edinburgh (until March 28)

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