I had a great day today. I shot three jobs, enjoyed them all and the pictures were pretty good. The final job of the day involved shooting a portrait of a senior Oxford University academic - something I do regularly. The difference with this one was that he had an opinion about photography.

The man in question is Dr Theodore Zeldin. He greeted me warmly at his front door and followed the pleasantries with a statement which went something like this:"You photographer chaps are all the same, you come in, get on with some chat for five minutes and hope to get the measure of me so that you can put what you have learned about me into your photograph. It doesn't work you know". Now, if that was calculated to wrong foot me, it worked. This eminent man is a philosopher, linguist and historian, he is also as mentally sharp as they come. I tried not to show that I had been rumbled and replied with a short speech about "photography as shop window" and about how my work as intended as "an addition to the written word". My determination to portray this man as I saw him grew.

As I set up my lights to match a whole set of other portraits we continued to talk about portraiture. Dr Zeldin was "utterly convinced" that the best a photographer can do is to examine the shape of the sitter, arrange them with their surroundings and get a good likeness of them.

I countered with examples of great portraits and his genuine familiarity with many of the images I mentioned was truly refreshing. I was beginning to think that he was a keen amateur photographer on the side, but he denied this. He was obviously well read and I realised that the best photographs have a life beyond their first use and, as someone who has devoured the quality press from across the world for decades, he would have seen these images as they were intended to be seen.

We moved out into the garden and I did my usual thing of lighting the portrait outdoors, letting the background go a little dark. We carried on talking about the world, about academic and political figures that I had photographed and about composition in art and photography. I started to talk about what you can learn as a photographer and about what you simply have "an eye" for. I mentioned some of my heroes and in answer to his question about who the absolute master of composition was I replied "Cartier-Bresson, no McCullin, no Doisneau, definitely Robert Doisneau".

He replied " Ah yes, he was here many years ago." I was utterly wrong footed this time. Here I was, thirty years on, shooting a portrait of a man who had been photographed by one of the true greats. He then reeled off a few more names - big names - and I started to feel confused. I must have looked a bit strange because he asked if I was all right. I said that I was, and suddenly started to feel part of something big - history. I often photograph people of this stature, but never before had I shot someone who had such a love of photography and a knowledge of great photographers - a personal knowledge. I've been accused of name dropping many times, but this guy had me well and truly beaten!

I love my job, and days like today make me realise why. Good variety, interesting people and great locations.