Wolfgang Tillmans, Serpentine Gallery, London

Looking at an exhibition by the German photographer Wolfgang Tillmans is a little like sitting down in the street outside a café in a city little known to you and watching the world go by.

What do you do? You look at some things more eagerly than others. You think thoughts random, philosophical or political by turns. You have a chance conversational encounter. It goes somewhere. Then, minutes later, nowhere. It is all quite casual, quite serendipitous.

And so it is here at the Serpentine Gallery. Shows here are usually accompanied by a certain amount of background information. Here, there is nothing. You plunge straight in. You don't find out, for example, that Tillmans once won the Turner Prize. It was at his Turner show that he first shocked his audience by displaying his photographs tacked to the wall or held in place only by bits of tape, unframed, unmounted. This habit continues, though he also shows some of the larger works in frames these days. That casualness of presentation is all part of the work's meaning – something is being captured on the wing here. He also puts obstacles in the way of our enjoyment of the work. The captions to the photographs are exceedingly brief, often in German, and sometimes fairly unhelpful. They are organised in groups, but in which direction are these lists to be read? Is it left to right or right to left? Does this particular caption cover several photographs or just one? Sometimes the captions to a photograph are not even on the same stretch of wall.

Tillmans show us the world of his ever shifting preoccupations. He documents what he sees in front of him, displaying the fruits of his interests in singular juxtapositions – an image of two idling athletes at a heptathlon hangs beside a crate of egg boxes, which hangs beside an image of a building. Sometimes, shifting rapidly from the macroscopic to the microscopic, he shows a keen interest in pushing photography's technical boundaries – he makes giant images of light itself, in his darkroom. These are the show's best pieces precisely because you never really know what they are until you ask one of the attendants – they look like tiny, combed filaments of something or other. It could be hair. Or blood. Does blood ever look like this? All we know is that we don't think that we have ever seen it before with the naked eye, at least not looking like this.

At other times, he is roaming the world, quite serendipitously, noting acts of social injustice – the Jerusalem Wall that divides communities, a photograph called Empire, which shows us the Mexico/American border or, in a couple of mixed-media, table-top installations, giving us little lectures about organised religion or the persecution of gays. These are not so interesting because it's far too obvious which side he's on. At other times, he is consumed by an interest in things elemental – an entire series of photographs are devoted to the planet Venus, for example, the last one showing Venus in eclipse. He photographs nature grainily, as if it belongs to a different century from this one. Some of the photographs are terrific. Many of them are no more than quite good.

To 19 September (020 7402 6075)