Nicholas Sinclair: I was on a train to London in 1991 and found myself sitting with the art historian David Mellor. We were chatting and he said to me, “If you could photograph any British artist who would it be?”

And I said John Piper. This was partly because I have always loved Piper’s work and partly because he had a very unusual, elongated and slightly haunted face with deep-set eyes that I felt would make a great portrait.

So David gave me his address, I wrote to him and he agreed to be photographed. When David saw the pictures I had taken he asked me to work with him on an exhibition he was curating at the Barbican Art Gallery in London about the artists who came to prominence in the 1960s. He wanted me to make contemporary portraits of these artists and this is how the series began.

I photographed Caro in his north London studio in November 1992. I remember a huge workshop, full of scrap metal and sparks from oxyacetylene torches, with sculptures in the process of being made and assistants busy welding. The atmosphere was very focused and productive and everybody seemed to know exactly why they were there. Caro agreed to give me his lunch hour for the portrait so I knew how long I was being given and this meant that I too had to be focused.

My first priority with any portrait is to find the right location. It is essential for me to find a background that works visually before I can begin to think about the person I am photographing and with the artists I wanted to include references to their work or their working methods or the materials they use so that each portrait would be informative on different levels.

For Caro’s portrait I decided to use a white wall with iron hoops suspended from a hook and when I showed him the Polaroid of the composition he liked it and agreed to work with it.

What makes this portrait slightly unusual is that Caro’s face occupies only a small fraction of the surface area of the photograph. But, partly because of the strength of his gaze and partly because of the way I have composed the picture, you have a very clear sense of the man. Regardless of where you are in the photograph your eye is always drawn back to Caro himself. The strong black vertical, two thirds in from the left, leads you down to his right shoulder. The hoops on the right of the picture lead you to his left shoulder. The hoop fourth in from the right leads exactly to the line of his eyebrows, and the hoop that curves down from the left just touches the top of his head.

And these suspended iron hoops above his head are a direct reference to the way Caro broke free from Henry Moore’s influence, did away with the idea that a piece of sculpture had to be seen on a plinth, and changed the way we now think about sculpture. So this portrait is a long way from just being a casual snapshot.

Yes it is, but it’s not always possible to find backgrounds that integrate so well with the subject. Sometimes I feel as though I’m being given a gift as a photographer, and Caro, on that day and in that studio, was a gift. My role was to recognise and respond to the situation by using my judgement, my graphic sense and my experience with light to make the image happen.

Always. It helps me to make a more substantial portrait. The sitter will often pick up on the fact that you know about their work and this will make a difference to the way they respond to you.

The artist plays a very important role in the process because I’m working in their studio and often including their work in the composition. The environment during the shoot is one they’ve created and I’m sensitive to this, but the choice of setting and the placement of the artist within that setting is down to me.

I will compose the picture, I will choose the subject to camera distance, I will select the aperture and shutter speed settings, and, most importantly, the moment of exposure, so it’s very much a collaboration and this is the way I have always seen portraiture.

Regarding poses, I prefer something natural and unstaged, but the subtleties of the pose have to be with the sitter themselves. I like poise and dignity in a portrait, both of which are quite unfashionable right now, and when I’m examining contact sheets I will often gravitate towards images that present the artist as thoughtful and serious and I’m unapologetic about this. I also look closely at the way the hands are portrayed because they can reveal a lot about a person.

The first thing I do is to assess the light in the artist’s studio. In the case of Frank Auerbach, Paula Rego, Josef Herman, Kyffin Williams and the more traditional painters, I was often able to use the natural light because they’ve chosen to work in studios with north light, which, because it has an even quality, is good for portraiture.

If the light is inadequate I use electronic flash which I either diffuse or reflect so that it’s subtle and sometimes undetectable. I prefer the technical side of photography to be more or less invisible so that the viewer goes straight to the subject and to the eyes and is not distracted by other issues. For me, everything about a portrait should support the gaze and should be subordinate to it, even if the eyes are partially hidden or in shadow.

I’m aware of a melancholic strain in the work because it’s been there from the beginning, but I don’t set out to create it. While I’m taking the portrait I will be looking closely at the changes in the person’s expression, at the subtle movements in the muscles around the eyes and the mouth. But it is in the editing process, which happens after the photographs are taken, that I will see these changes more clearly and it’s only then that I can select an image that I feel expresses something about the sitter.

It’s a sense of engagement with the subject. For me this is crucial if the picture is to have any lasting value. It’s the same in the cinema, in the theatre and in literature. If there isn’t a connection, however subtle, then the portrait has little or no meaning, so I’m looking for that moment when, either consciously or unconsciously, the sitter reveals something about themselves that the viewer recognises and can relate to.

Yes I do. The vast majority of my portraits are in black and white. I like the sense of austerity you get in a black and white print. I like the way it handles skin tones and there is something about the quality of a silver print that I have grown to love. There is also a long tradition of photographing artists in black and white and there’s no doubt that those wonderful Hans Namuth portraits of the American painters of the 1940’s and 50’s, Jackson Pollock, Franz Kline etc., have had an influence on me.

But when Anthony Caro died last year [2013], and when the Guggenheim Foundation wanted my portrait of him for their Venice collection, I looked again at the portraits I had taken and I found some colour pictures, not just of Caro but of Fiona Rae, Albert Irvin, John Hoyland and Gillian Wearing, so we decided to include some of these in the exhibition. They’ve never been seen before, not even by the artists themselves, and we felt that it was an opportunity to show them for the first time.

There’s a long list of people I would very much like to photograph. Sean Scully is one. Sarah Lucas is another. Tony Cragg and Richard Long are two important sculptors I haven’t photographed yet and there’s a new generation emerging I must photograph if the series is to feel comprehensive. I also want to see more women represented because they have been underrepresented in British art until recently.

Strong characters with atmospheric studios who make great work are what really inspire me, but they are far harder to find than you would imagine. I knew when I was photographing Frank Auerbach and Anthony Caro that these were special moments in my career.