Christ gestures ahead while turning back to the brothers Simon (later Peter) and Andrew, who have been fishing by the Sea of Galilee. He says 'Follow me and I will make you become fishers of men'. The fishermen's weather-beaten faces and hands eloquently express astonishment, confusion and fear.

The subject of this painting comes from St Mark's Gospel (1:16-18): Jesus saw Simon (later called Peter) and his brother Andrew fishing by the Sea of Galilee and said, 'Follow me and I will make you become fishers of men'. A young Christ, unusually shown without a beard, gestures out of the painting in the direction in which he is moving, while turning back to the two brothers. Peter holds two fish skewered on sticks and starts back in surprise; Andrew points to himself incredulously. The composition seems to imply that Christ will walk calmly off to the right, without looking back again, and that the other two figures (after a due period of astonishment and reluctance) will find themselves following.

With a few exceptions, scholars have believed that the Royal Collection picture records a Caravaggio design, which is also known through eleven other versions. All twelve versions were thought to be copies of a lost original until, in 1987, Maurizio Marini suggested that the Royal Collection version might be that original, a view that has been widely accepted. Recent conservation of the painting has helped to resolve the question.

Examination of the painting revealed that an expressive furrowing of Christ's brow was not part of Caravaggio's original idea, but had been caused by damage to the paint surface (or rather by the discoloration of the early touching out of the damage). This 'frown' occurs in two versions of the painting, at Weston Park and formerly Chatsworth, which were clearly copied from the Royal Collection painting, probably in the late seventeenth century. Conservation has also revealed the dynamism of the composition: Peter's right arm, which previously read as flat, now seems to project out of the front of the picture plane (thrusting the fish under our noses) in an effect typical of Caravaggio. The relationship between his two hands and the figure of Christ establishes a diagonal recession into space, which was illegible before the painting was cleaned. We can also now appreciate the contrast between Christ's long, pale fingers and the weatherbeaten hands with which these working fishermen express their surprise, confusion and fear. The face of Christ and his hands are the most fully lit of the scene, while those of his disciples are in more shadow. The paint was applied swiftly but accurately, though Caravaggio made numerous small but significant changes as the composition progressed. Also characteristic of his later work is the use of the priming colour as a middle tone, visible in the completed work - in this case on the fingers of the figures, and between areas of colour in Christ's greenish-blue drapery.

No drawings by Caravaggio have been identified, and it is likely that he painted directly onto the canvas. To help him he developed the unprecedented method of incising the lower paint layers (perhaps with some form of stylus) to mark out the important positions in his composition. Many such incisions are visible here, around the ear of Andrew, covered with hair in the painting, and mapping out crucial lines around the head of Christ his eyebrows, shoulder line and lower sleeve. The blue of Christ's robe has been seen as an argument against the attribution: according to Bellori, Caravaggio did not use cinnabar red and azure blue, 'and if sometimes he may have chosen them, he softened them, saying that they were the poison of colours'. The blue used here is certainly unusual for Caravaggio, although he uses blue in the Baptism of Christ (National Gallery of Ireland) of c.1603 and the Annunciation (Nancy) of c.1604-5. However, restoration has revealed that the colour was modulated to a rich greenish blue in the highlights but a dark almost 'indigo' colour in the shadows, which perhaps qualifies as the type of 'softening' Bellori mentions.

Placing this work within Caravaggio's career is hampered by the fact that there is no record of him having painted such a subject. For many years the composition was linked to a painting described by early biographers Giulio Mancini and Giovanni Baglione as a 'Christ who goes to Emmaus'. It clearly does not depict the pilgrimage to Emmaus - an episode in which fish play no part - though it is just possible that both authors misread the subject. This 'Christ who goes to Emmaus' is now generally thought to be a reference to the Supper at Emmaus of 1601 (National Gallery, London). Without documentary anchors the Royal Collection painting can only be dated stylistically and seems to belong in the period 1603-6, painted in the years before Caravaggio fled Rome, having killed a man in a duel in May 1606. Compared with the Supper at Emmaus the technique is economical, the brushwork broad, the colour restrained and the detail minimal. In Caravaggio's art during the years 1601-6 sensuous surface detail gives way to a spare, dark and expressive manner. The technique here is closer to the Supper at Emmaus of 1605-6 (Brera), in which Christ again wears a greenish-blue robe. It can also be related to two other compositions where a small group of half-length figures interact intensely against a dark void: the Doubting Thomas (Sanssouci, Potsdam) and the Betrayal of Christ (National Gallery of Ireland, Dublin), both of c.1603.

The only other hint as to the history of this painting before it reached England is provided by the interesting early copy of the painting by Bernardo Strozzi (private collection) in which the effect is lighter and a lobster is added to Peter's catch. Strozzi worked in Genoa until his move to Venice in 1630. He may have seen Caravaggio's painting on a visit to Rome, but it is also possible that it had found its way to Genoa, perhaps in the collection of one of Caravaggio's patrons, such as Vincenzo Giustiniani, Ottavio Costa or Prince Marcantonio Doria, who had connections with that city. It was in 1637, only a few decades after its execution, that the painting was acquired for Charles I by his agent, William Frizell. As well as working for the King, Frizell assisted William Petty in various tasks in Italy in Lord Arundel's service. We know that he landed in Livorno and must have travelled to several Italian cities in 1637, including Rome and Naples, though there is no record of his visiting Genoa. The painting appears in the Royal Collection inventory of c.1639 by Van der Doort as 'the 3.disciples Comeing from fishing said to be don by one at Room who is an - Immetator of Caravagio'. The phrase 'at Room' could imply that Frizell bought the painting in Rome. It was listed at the Commonweath Sale as 'thre Fisher men. done by Mich.Angelo Cororagio'. Horace Walpole annotated the entry in his copy of the James II 1688 inventory with 'It is now over a door at Windsor, & is one of the finest pictures the King has'.

Catalogue entry adapted from The Art of Italy in the Royal Collection: Renaissance and Baroque, London, 2007