Some of Edward Hopper's paintings such as "Nighthawks" (1942) and "Automat" (1927) have become icons of 20th century art. This is perhaps one of the greatest honours that can be conferred upon an artist. But the omnipresence of his work in the form of posters, calendars and coffee mugs also makes it difficult to approach the original paintings with an open mind.

Edward Hopper was a painter. This may sound like stating the obvious, but even at the time when he was working, if he had just been after an image, Hopper could have taken a photograph. This is what numerous fashion editors, stylists and photographers have done by staging the scenes in his paintings. But even if all elements were to be arranged in precisely the same manner there would still be a difference between a photo and a painting of, say, "A woman in the sun" (1961) or "Automat" (1927). The difference is in the details. A photo, at least of these compositions, will always show too much detail. Hopper paints a chair, a table, a woman, a desk, whereas a photo will show this chair, this table and that woman. In Hopper's paintings the emphasis is entirely on the scene as such.

Because there is not much detail in his paintings, the rooms could be any room, the streets any street and the persons anybody. It could be you as a matter of fact. And this, I think, is one reason why his work strikes a chord with many people. The woman sitting behind a table in "Automat" (1927) could be you or me waiting in the station buffet, when you just missed your train and you have to wait another two hours before the next one. I know what I'm talking about. I've been there. This is also why I find paintings such as "Nighthawks" timeless rather than nostalgic. After all, it could be a retro bar in New York, London or Tokyo.

Hopper is often described as a realist painter, but I find the term abstract realist more appropriate. He does not represent a scene. He renders an experience: those empty minutes in a theatre before the curtains go up, an hour of the day, early Sunday morning.

People rarely move in Hopper's paintings. They sit or just stand there, like "A woman in the sun" (1961). Quite often they are reading, a letter, a memo, a book or a newspaper. It's one of those things you do when you've got time to kill, but it is also a highly solitary activity that demands concentration and can make you forget your surroundings. As in the work of Vermeer you wonder what is being read. The mystery is in what we don't see.

It is hard to look at a painting by Hopper and not to think something. The perspective in his paintings is often voyeuristic. We see the figures through a window, but in paintings such as "City Sunlight" (1954), "Cape Cod Morning" (1950), "Office in a Small City" (1953), "Room in Brooklyn" (1932) and countless others, these figures, too, are looking through a window, which makes you wonder what they are looking at and if they are looking at all or merely staring into space.

Hopper was not the most technically gifted artist. His figures always look a bit awkward. His landscape paintings of a house in Cape Cod or a lighthouse would be unrecognizable if placed among some amateur paintings. It is when he introduces people in his work that Hopper's paintings become most distinct. It is their presence, which makes the places appear empty, such is the logic of his painting.

And yet, intriguing as some of his images are, apart from their size, I couldn't help thinking that the actual paintings add little to the images. There is nothing special about their texture, except that his paintings are mat whereas reproductions tend to be glossy. So if you want to know what you've missed, just browse through one of the many books on Hopper in your local bookstore.

Edward Hopper is at the Museum Ludwig, Köln, until 9 January 2005.