Claude Lorrain: The foreign fields that inspired our greatest landscapes

Turner worshipped him, Gainsborough argued that there was no need to paint real landscapes when you had him as an inspiration, and Constable declared him quite simply "the most perfect landscape painter the world has ever seen".

Without the 17th century Lorrainese artist, Claude Lorrain, that most quintessentially British of all art genres, the landscape, might never have developed, or certainly never grown in the way it did. Parklands were laid out in imitation of his views of the Italian countryside and, at one time, something like two-thirds of all his paintings were in Britain.

And yet if he is known today in this country, it is mostly as a name rather than as a painter of loved works. The pre-Raphaelites rejected him, the Modernists ignored him and, even now, when his reputation has risen yet again among scholars, to many ordinary art lovers he seems a figure of the past, all those small figures set in idyllic countrysides with their references to mythological history.

In launching a major exhibition of his works, the Ashmolean Museum in Oxford aims, as the curator, Dr Jon Whiteley puts it, "to free him from a reputation seen through the eyes of later painters" and restore him as an innovative artist who carved out a place for landscape in a particularly individual way.

They are doing it not just by displaying some of his great paintings, exhibited in pairs as he intended, but by accompanying them with an extensive display of both his drawings and his lesser-known etchings.

The drawings, and the etchings, in particular, prove the point of his individuality. Claude Gellée , his real name, was never a formerly trained artist nor an academic one. Born in poverty in Lorraine, orphaned at the age of 12, he first became interested in art, according to one of his early biographers, when he entered the house of an Italian painter as a pastry cook . It's too good a story to be dismissed, although other accounts say he first developed his interest helping his brother woodcarving. Like his French contemporary and friend Nicolas Poussin, he fell in love with Italy, its ruins, its past and the gentle landscape of the Roman campagna. But, although he went on drawing expeditions with the great Frenchman and remained a near neighbour in Rome, the contrast between the two is instructive. Poussin was cerebral and literary and drew clearly and precisely with the final painting in mind. Claude drew in a notebook with pen, ink and chalk and wash en plein air whatever took his fancy and then elaborated the drawing as a sourcebook for later painterly concepts.

His early works are full of the spontaneity of discovery as he studies an ancient ruin, captures the rhythm of a river or the pattern of a clump of trees. Above all he was fascinated by light and the atmosphere of the time of day. His earliest biographer recalled him, somewhat comically, lying on his back determined to record the precise light of the moment. But that was what captivated him and it shows in all his works. There's a couple of wonderfully fresh studies of trees and shrubbery from 1640 done with such a fluid wash that you can almost feel the moment. Other drawings done with pen and brown ink give the lie to the view that Claude (as was said of Turner) had no way with the figure.

The other point made strongly by the drawings and etchings is just how conscientiously Claude built up his repertory of compositions and details as he developed his reputation and moved up the scale of patrons (Pope Urban VIII was an early admirer). In his day he was known for copying all his paintings into a liber veritas. The aim, he said, was to defeat forgers by producing a precise record of the works he sold. In fact he used it continuously as a source book of ideas, using images from previous paintings and reworking them for later paintings. He did the same with his drawings and his etchings, and the variations and changes he made are fascinating.

The Ashmolean is showing all 40 of his known etchings and they form a fascinating sequence. Claude etched in bursts and produced a number in groups. What comes through is his use of multiple biting of the plate to produce atmosphere. The Cowherd from early on his career is a true masterpiece of poetic mood but so is Departure for the Fields from 1640 and The Herd Returning in Stormy Weather from a decade later.

But if the drawings and etchings give you a closer understanding of just how carefully Claude worked to achieve the extraordinary sense of the idyllic scene which so impressed his contemporaries and later artists, it remains the paintings themselves which can still knock you out. The Ashmolean, which has organised this show together with the Städel Museum of Frankfurt, has assembled a dozen of his greatest masterpieces, four of them arranged in pairs – land and sea, morning and evening, the light from the left and the right. Standing in the middle of the room and looking around you is an overwhelming experience.

Constable called the unique sense of peace and beauty Claude achieved "the calm sunshine of the heart'. But it was Turner who understood him best and who was the most profoundly moved by him (an exhibition called Turner Inspired: In the Light of Claude is due to be held by the National Gallery in March next year). It is the way he paints the sun and light itself which is so extraordinary. The pair of Pastoral Landscape with the Arch of Titus and Coast Scene with Landing of Aeneas (both from private collections) beggar belief in their capture of light and the way it glows through the trees but then this is also the case with the National Gallery's Landscape with Psyche Outside the Palace of Cupid in which the light is almost white and ghostly, and its A Seaport from 1644, in which the sun itself is pictured in all its glory. Turner was right. It was not the landscape but the sunlight that suffused it which was Claude's greatest contribution to art.

Claude Lorrain: The Enchanted Landscape, Ashmolean Museum, Oxford (01865 278000) to 8 January