An early triumph of pleinair painting, Monet’s ‘The Beach at Trouville’ (1870) has grains of sand blown into the wet paint

“There is only one master here – Corot. Compared to him, the rest of us are nothing.” Thus Monet, on the quiet revolutionary who is the unlikely star of the National Gallery’s splendid, free summer show Corot to Monet: A Fresh Look at Landscape from the Collection.

Born the son of a wigmaker and milliner in Paris just before the revolution, Jean-Baptiste-Camille Corot pursued an ancien régime classical training in Rome, then spent a lifetime depicting everyday landscapes in the Ile de France and Burgundy, opening French painting to naturalism and a freshness of experience without which modern art could not have happened. “He is still the strongest, he has anticipated everything,” Degas said, while, for Matisse, as late as 1912 there were only two 19th-century masters: Manet and Corot.

Popular taste ever since has disagreed. This show is the latest of attempts that began in the 1960s to revive Corot and the Barbizon school as more than candle-bearers to impressionism. Drawing on the gallery’s extensive holdings of small, glowing oil sketches – many rarely displayed – and a few celebrated canvases, it is a triumphant balancing act, managing to establish a continuum between early and later 19th-century French painting without diminishing the shock of the impressionist breakthrough.

The story opens with Pierre-Henri de Valenciennes (1750-1819), who painted dry Roman vistas of light and shadow under big monochrome skies, as in “Cow-shed and Houses on the Palatine Hill”, and closes with Monet (1840-1926), whose “Bathers at La Grenouillère”, depicting pleasure-seekers at a rowdy river resort in rapid, fluid brushstrokes, thrills with the pulse of contemporary life and has the freshness of a plunge into the Seine. The evolution from stiff heroism to spontaneity is marked, yet it was de Valenciennes who encouraged plein-air painting from the motif, and insisted on landscape as a valid subject – rather than mere backdrop to mythological scenes, as in Claude or Poussin – for the Paris salons.

Once this was accepted, precisely observed homegrown motifs began to replace classical idealised ones. The simple, backlit “A Tree” (1816) by Corot’s teacher Achille-Etna Michallon contrasts an old hollow oak trunk with young branches and leaves. Paul Huet painted “Trees in the Park at Saint-Cloud” (1820) with calligraphic swirls and loops. By 1830 the forest of Fontainebleau, centred on the village of Barbizon 30 miles – but eight hours’ travel – from the capital, was an open-air studio. Théodore Rousseau came to know its trees as individuals, likening the experience of seeing them chopped down to an “amputation”. His black crayon and thinly painted oil on millboard “Landscape” (1865), with skeletal winter trees, pink-tinged clouds and fading daylight, is a high point.

“Never lose the first impression that moved us” was le père Corot’s insistent advice. His youthful classical studies – sunlit views of aqueducts and ramparts against broad southern horizons such as “The Roman Campagna” (1826), which belonged to Degas, and the lovely “Avignon from the West” – already show his gifts for simplification and crystalline tone. These underline, and save from sentimentality, his later imaginary vistas in silver-blue-grey, where exquisitely composed landscapes of trees, misty lakes, a distant town, appear as diaphanous memories, light shimmering through a lattice of branches and leaves as if through a veil.

The tangled foliage in “The Leaning Tree Trunk” of 1860-5 echoes sketches made at Civita Castellana, north of Rome, 40 years earlier. The unassuming “Peasants under the Trees” transposes classical order to the peace of a provincial scene: silhouetted against the sky in cool morning light, a peasant couple saws timber; long shadows fall; a glint of white draws attention to a waddling goose; perfect-pitch notes of grey, brown, green tinged with blue are warmed by a single red fleck on the woman’s hat. Corot’s pivotal, double-edged genius was to channel a personal experience of nature within a classical form, yet always one feels in his nuanced colours and dancing highlights the impermanence of the harmonies he achieves so tenuously: if his fleeting effects anticipate Monet, his heartbreak and staginess also recall Watteau.

Though loved by fellow painters, Corot’s progress through the salon was blocked at every turn by the Count of Nieuwerkerke, head of art patronage during the Second Empire. He dismissed the Barbizon school as “the painting of democrats, of those who don’t change their linen, and who want to put themselves above men of the world”. One of the pleasures of this show is its illustration of how through the mid-19th century there was no single sustained assault on the stranglehold of salon academicism, but rather a diversity of personal, deeply-felt approaches, from the glower of Courbet – “Beach Scene” is mostly thunderous grey sky, built up from slashing strokes – to the refinement of Daubigny’s play of light on the Oise and Seine such as “River Scene with Ducks”; extra ducks were dabbed on, apparently, to those canvases Daubigny preferred, and this one has a flock.

In metropolitan contrast are Eugène Boudin’s many versions of “Beach Scene, Trouville”, which brought Paris to the sea in a froth of parasols and sashes, and introduced Monet to painting outdoors. Monet’s own bravura “The Beach at Trouville”, with grains of sand blown into wet paint, is an early result: wind seems to rush across promenade, marquee, flag, while Mesdames Boudin and Monet – denoted with extreme economy of strokes including just one squiggle of paint for the ruff of her dress – recline dreamily in the foreground.

This is an intensely French show, a story of the continuities and changes between Corot’s arrival in Italy in 1825 and the first impressionist exhibition in 1874, during which Paris supplanted Rome as the world’s cultural capital. A generation later, when the city became the laboratory of modernism and hundreds of foreign hopefuls converged on Montparnasse and Montmartre, it was the unrelenting Frenchness of Parisian painting that most struck the new arrivals. “Everything showed a definite feeling for order, clarity, an accurate sense of form, a more painterly type of painting even in the works of lesser artists,” noted Marc Chagall, trying to explain “the almost insurmountable differences which, up to 1914, separated French painting from the painting of other lands”.

This scholarly, unflashy show dramatises how the solid achievement of half a century of French painters was a crucible in art history, and gloriously brings to life an often overlooked part of the National Gallery’s collection.

‘Corot to Monet’, National Gallery, London WC2, to September 20. Tel: +44 (0)20-7747 2885, www.nationalgallery.org.uk