Jeff Wall, White Cube, Mason's Yard, London; Seb Patane, Tate Britain, London

A Jeff Wall show used to be a straightforward event, until he merged his cinematographic skills with a new love of documentary

We used to know where we were with Jeff Wall's photographs. The Canadian artist had studied at the Courtauld, we were told, and his panoramic images were conceived on the same scale and with the same level of detail as the narrative paintings of 19th-century masters like Courbet and Delacroix. He agonised, we believed, over the casting of actors and the composing of a scene in the manner of Martin Scorsese, before taking just as long to produce the finished work as it did to make Gangs of New York. Finally we were left to marvel at the sheer endeavour when the pictures were displayed like illuminated airport adverts on the gallery wall.

A few years ago, however, all this changed when it was announced that Wall, 61, had discovered documentary photography. Rather than spending weeks setting up his photos, he found that he could shoot ready made scenes and present them as if they were as carefully conceived as his works in cinematography. As a result, it's not so easy to understand Jeff Wall any more. When we look at the three new illuminated photographs on show at White Cube, for example, are we looking at his carefully stage-managed images or his blown-up snapshots? And what about the black-and-white photographs downstairs, the ones he calls "near documentary"? Are there any clues to help us decipher what is real and what is not?

There's something obviously repellent about Dressing Poultry, the huge photograph of women plucking chickens which dominates the main gallery, and yet it's hard to take your eyes off it. We are gripped by the laughing face of a worker and compelled to consider the contrast between her expression and her action as she pulls the entrails out of a chicken carcass. A fellow worker chuckles with her, but what are they laughing at? They are standing in a warehouse packed full of poultry feathers, work tables and miscellaneous objects like bicycles and coke crates. On the floor there are pools of coagulating blood.

In terms of subject matter the photograph confirms the artist's interest in the French realist tradition of depicting low-paid workers toiling at their jobs. And like Manet or Courbet, Wall takes trouble with the lighting and the composition to make a picture in which the beauty and humour is in contrast to the unsettling subject matter. But how staged is the scene? Wall has said these pictures are documentary images but it's hard to trust someone who has made their career revelling in layers of artifice. This chicken-plucking place may exist, but there are clues that suggest that the artist hasn't just stumbled in; how else can you explain a cardboard box in the foreground conveniently stamped with the word "Grimm's"?

The two other backlit photographs are less tricky to decipher. Hotels, Carrall St, Vancouver, shows a row of tall Victorian buildings in the process of renovation. This scene, together with Church, Carolina St, Vancouver, which shows a clapboard building signposted as a Slavic Pentecostal church, seem to be genuine observations of a city in the process of change rather than stage-managed events. The compositions are as considered as Hopper's and the works are as satisfying as aesthetic objects as they are as social comment.

Six large black-and-white photographs are displayed in the second room and again we must question their appearance as documentary images. For example, were the boys behind the barricade in War Game really holding up plastic guns as if practising for some future war, or has Wall conscripted actors and gone out on location? It's hard to tell. Where there are no humans as in Cold Storage, things seem more straightforward. Here a concrete bunker contains nothing but an icy ceiling and a frozen patch on the floor. We don't know what has happened here, but the scene is chilling in every way.

If Wall's new pictures were really documentary photographs, they would surely lack the gravitas of his older pieces. But the doubts we have about whether we can trust him laces the experience of looking at his work with a whole new thrill.

In stark contrast to the high gloss of Jeff Wall's prints is Seb Patane's installation, This Song Kills Fascists, on show in the Art Now space at Tate Britain. This thirtysomething is a graduate of Goldsmith's MA course and one of the new breed of artists whose work appears wilfully badly mad, as if in protest against the rise of art as luxury goods. The installation features a series of framed drawings scribbled in ballpoint, some screenprints of protest marchers, and an excruciating soundtrack of the title song which makes your head hurt. His project, apparently, is to investigate how the subculture of art and music became detached from political action. It's a worthy cause, but the installation is so incoherent that it ends up saying nothing. Do take time to watch the amusing video interview though: Patane, rather loftily, feels that the focusing on the artist rather than the work is suspect and therefore opted to have a hypnotist appear on camera to read out his answers for him.

Jeff Wall, to 19 January (020 7930 5373); Seb Patane, to 13 January (020 7887 8734)

Further reading 'Jeff Wall: Modern Artists Series' by Craig Burnett (Tate Publishing)