The secret diary of Mario Testino: What the world's most glamorous photographer saw

He is one of the best-known photographers on the planet, turning models into superstars and criss-crossing continents to shoot princesses, pop stars and Hollywood players. So just what is the secret of Mario Testino's A-list allure? In an exclusive three-week diary, he takes us behind the scenes of his glamorous life

May 29

Fly into Düsseldorf to control the printing of an exhibition I'm having at Phillips de Pury Gallery in London this summer. Grieger is the only lab in Europe that specialises in large-format printing. You walk in and you see the work of Andreas Gursky and all the other German artists that I adore. I had to stand on top of a ladder to see them because they are so big; it's impossible to hold them up.

The exhibition includes pictures from all aspects of my career (left). But I try to keep the royal clients out of it. When I did the Diana exhibition I asked her kids, Prince William and Prince Harry, to come see it before it opened. I've been very careful with those pictures. A lot of people could have sold them everywhere and made money. But as much as it's very exciting to tap into their lives, I am just a photographer who happened to photograph her for one day. I have no right over those images. I am very close to my mother and I would hate anyone to do whatever they wanted with her photograph.

[Prince] Charles and his kids have an amazing relationship. I noticed this when I first did their Christmas card, and I exploited it – the way children always tease their parents. I kept saying, "Harry, William – go and tease your dad," and he was, like, "Mario! You're supposed to be on my side!"

I adore every step of the English. I like Prince Charles as much as the street cleaner. I don't make distinctions on rank. I love those [London] Tube ladies who say "Ta-ra, love!" I used to think, "How great that they know me." Then I realised they say "Ta-ra love!" to every single person who goes by.

June 1

Off to Berlin for the launch of an issue of German Vogue. I was asked to edit a whole issue, and photographed it all myself. The only thing they imposed on me was a theme: sex. That was not an unknown subject for me. My all-time inspiration is Helmut Newton, a German, who happened to be born a day after me – though not the same year. We are both Scorpios and people often say to me, "Scorpios are very sexual". Maybe they're right, because sex does recur in my work. I grew up in Peru in a very conventional Catholic upbringing, so sex was a taboo. But at the same time it was the 1960s; the sexual revolution. So I'm a contradiction. I came to London when I was 21, arriving in 1976 – the hottest summer in history. I was amazed so many people were naked. I would go to parties and they were all naked! I'm a South American and South Americans do grow up in their bathing suits. From the age of 14 I spent most of my summer holidays in Brazil in a bathing suit. They wear the smallest suits you've ever seen. I still wear my Speedos when I'm there. I don't look that good, but if you wear something else you look like a freak.

German Vogue throws a party for me – a dinner for 200 at my favourite restaurant in Berlin, the Grill Royal. Everybody from the German scene comes: [supermodel] Nadja Auermann, [artist] Jonathan Meese, [designer] Wolfgang Joop to museum curators to musicians. It's a really happening scene.

I did the cover of German Vogue (above) with Claudia Schiffer because, to me, she's the all-time German export. At the start of her career she was portrayed as a Bridget Bardot remake; she was this young girl who was so innocent she went everywhere with her mother. But now she's got her children, she's a woman. I wanted to portray that new confidence.

I also photographed this new girl, Toni Garrn. She's 16 and may be the new Claudia Schiffer. She was also picked up by Calvin Klein, but when I saw their campaign I thought it looked... I thought you didn't really get her. A lot of campaigns transform the girl into what the designer wants. But when I met her I was so taken with her I was, like, "Oh my God, Toni!" So for this issue I wanted all the girls to have a starring role. It was about them, not the designer. That's why I put all their names [as the headlines to the fashion stories].

June 2

Location-scouting for my shoot with Keira Knightley (below) for the September cover story of American Vogue. The idea is that Keira comes to be inspired by the Berlin arts scene. Instead of galleries, I did it around artists' studios: people such as Daniel Richter, Thomas Schütte, Andreas Hofer, Thomas Zipp, Thomas Helbig. They all made themselves available and the contrast between Keira and them was quite interesting. She'd never heard of any of them! It's funny how little the art world crosses over with the film world. I'm really lucky because I have access to all these different worlds: the art world due to my curiosity, the film and fashion worlds due to my business, society and royalty due to the requests I have for work. I feel very privileged.

June 3-June 5

Three-day shoot with Keira. We move around everywhere; all over Berlin. At the same time I'm doing photos for my new book on the arts scene. So I'm multi-tasking!

June 7-June 8

Shooting in LA for an issue of V Man I'm editing. The theme is "extreme". I've tried to do it around the extremes of how a male gets perceived today, in the sense that there is ambiguity: sometimes the butcher a man is, the queenier he can be. And sometimes the most feminine men can be the most masculine. I'm influenced by so many different things, though. That's good if you're in the fashion business. Because things change so much. You can't say "I like colour" because the day after tomorrow black and white is in and you're out. And I don't want to be out. I need to work. It's my livelihood. So I'm adaptable.

Most of my work is for American Vogue. I shoot for it almost every issue. This is a magazine that sells 1.2m copies and is probably read by 3m. Its reach is so wide you have to be quite easily understood. Whereas British Vogue's aesthetics are different. You can't give them work that's too bland. It's got to have a certain humour; a certain fun. And my photography is always about finding the common ground. Me, I'm a democrat. I like to believe that of the 10 people at a shoot, from the manicurist to the hairdresser to the make-up artist to the model... if everybody likes something I get the idea that by the time the photos come out everybody will like it. If only three people like it then by the time it gets to the press, the same percentage will like it: a very small number.

A lot of people look at photographers in awe. But a really talented photographer is able to listen and capture what people are telling them. Some people are so closed they don't want to hear any opinions. They have people behind them thinking, "That looks hideous". But those people don't air their opinions, then they go home and go, "Well, I earned my living. But it was so bad." I choose to work with people who will share their opinions truthfully. Because I've had it in the past when people tell me, "Oh, it's amazing." And I'm saying to myself, "It's hideous. What are you thinking?"

June 9

Fly to San Pedro to do the Michael Kors campaign with Carmen Kass. I've been working with them for six years. I love commercial work. I found that, in the 1990s, advertising ' campaigns became more exciting than editorial. When I started doing Gucci with Tom Ford he pushed me to new heights. He was, like, "I've seen you do better than that. Don't get worried because it's a campaign." Before we were restricted because of the concerns around the world: you can't have nudity in some countries; a man cannot be touching a woman... there were lots of different things. He really changed things [with overtly sexual advertising].

It's so foggy that twice the plane tries to land and can't. That's nerve-wracking when you are the photographer; not because it's dangerous but because the moment you accept a job, the responsibility is on you; if the photograph is not delivered, the only name that lingers is yours. That has its pros and cons. Pros in the sense that you can be at a shoot and an idea can come from the secretary or manicurist and you can benefit from it: when people remember the image, they don't know the manicurist gave you the idea. But it happens the same way when things don't go right. Anyway, the plane lands and we do the pictures. And Michael was very happy with the campaign.

I'm usually responsible for the casting. I've reached the point in my career where all my clients rely on me being very involved in the process. Some photographers work under the guidance of the art director: they will sell the idea to the client and then book the photographer. My relationship is different because I mostly work directly with the clients.

And I love casting. I have my agency, who do research for me now and send me who they think I might like. I used to do it all myself, but there are so many models now I would never finish. It's the young models who really dictate what is happening in fashion. We all try to be fashionable, but fashion is for the young – the fashion world is not about working for senior citizens. That's what keeps photographers fresh, the idea that we find what's happening at the moment.

I find a lot of the best young girls come from [British model agency] Select. Every season for Burberry [Testino has shot the British fashion house's influential campaigns since 1998] I look to see who's new. Who is coming with something? When I noticed Agyness Deyn [who joined the Burberry campaign in 2007 with much fanfare, though has been dropped for the current push] she was this 17-year-old girl. I said "I really like her." I started working with her when I was doing a "teenager issue" for British Vogue. Because the Burberry campaign is about the youth of England, I have to change the cast to make sure it doesn't get boring. And Agyness is someone that the press have picked up on such a lot. These days the media is much stronger [at promoting people].

But I will still work with Agyness. I just did something with her in New York for a big issue of American Vogue, which comes out this month. It was with the cast of Hair. It was hysterical because that cast are so crazy and she's, like, the minimalist. Her look is so British. It's so stylish, my God! And it's totally her thing. I mean, it's her who inspires the stylist! Kate Moss has that amazing quality with her style as well.

I have a historic thing with British models. I have Kate, Jacquetta Wheeler – I started her – Lily Cole, Stella Tennant, Karen Elson, Erin O'Connor, Jodie Kidd, Sophie Dahl... British girls are the best for style. It's interesting because when I started in England, I formed part of the English pack. There's a generation of people in the fashion business who move from city to city: in the 1980s we were all in New York, then we were all in Paris. But wherever I go it's not like I want to be hanging out with the French or Italians! I hang out with the English. I'm not English, but I am in that creative way.

June 10-June 14

Three days off. We've just finished for the [fashion] season and you have to work every day because twice a year everybody wants to shoot on the same day and everybody needs the pictures on the same day, because of magazine deadlines. It's just work, work, work, work, work. Then you explode. So I have a small place in Los Angeles because I found I couldn't stand the hotels anymore. I lived in the Chateau Marmont [Hotel] for 17 years and I just couldn't take it any more. You need somewhere at least where you get up in the night to go to the toilet and you know where you're going. I hate hitting my head on walls.

When I take time off, I visit art galleries. I'm in awe of the freedom artists have. A fashion photographer has to produce a minimum of five pictures a day. There's no "Oh, I didn't like it, so I can't give it to you." The pages have been reserved, the money spent, so you have to produce. Also I do studio visits because I love to meet the artists. I once had dinner with Sadie Coles, Richard Prince and Elle McPherson (above) and Elle said something to me that really stuck. She said, "I never buy anything without meeting the artist." And I was, like, "My God, you have a luxurious time because lately I'm finding there's such demand that if you're not quick, it's sold."

Sadie was the person who got me collecting art. She's the guru. She criticised me a lot for what I was buying at one point: "What the fuck are you buying? Have you not got eyes?"

I also party a little bit. I like going out and meeting the young scene in each city. I'm 53 and I ain't getting any younger. Most of my savvy comes from the younger people. I'm lucky because what is happening to me now is that all the kids of my friends are now in their twenties so they can show me what's happening. And today there is an unbelievable openness between parents and children. I grew up in the generation where people would never open up to their kids: my mother and her sisters, when they were with us kids, they would speak back to front so we wouldn't understand. Whereas today, a dad can hang out with the kids and it's great.

Plus, I'm deprived of my family in the sense that my mother and sisters live in Peru or America, so these kids [of my friends] have become like my family. If I were to have an accident, they would be the ones picking me up from the floor. But one of them did say to me the other day, "You fool, Mario. You're old." I realised it was a joke. But I did almost kill him on the spot.

June 18

New York to shoot the cover for V Man. A young actor called Chase Crawford – he's, like, the heartthrob of the moment. It's much more unusual for me to photograph guys. I find women are more exciting because you can play with the fashion. With men, there are more restrictions.

June 19

London for Karaoke With The Stars, an event organised by Bella Freud for the [refugee] children of Palestine. It's amazing. Lou Reed is there, Bryan Ferry, Sharleen Spiteri, Lulu... and Gwyneth Paltrow sings. She does "Killing Me Softly" by The Fugees. It's beautiful. She's one of the people I am most attached to outside of business. Her and Demi Moore. When everybody's sung and all the stars have left and I stay to have a drink, who am I with? All the kids of my friends, of course. Bryan [Ferry]'s son, Daphne Guinness's son...

June 20

I leave today for Ibiza. I was a big party person until I hit 45. Then I started to think I looked like the bouncer. I love partying, but I have to limit myself to private parties now. My mother always says to me, "Your life is full of airports!" But I want to hit the beach and sit in the sun. I come from the period where people get properly tanned.

Mario Testino: Obsessed By You, to 26 July, Phillips de Pury Gallery, London SW1, 020-7318 4023