Historians have divided his work into periods, but the points of division are often vague, as he often completed a picture years after he began it. In his early period, he painted traditionally and "tight"—with minute exactness, clear outlines, thin brush work, and with absolute definition of objects throughout, with a monochromatic underpainting or ébauche. After he reached his 50th year, his methods changed to focus on breadth of tone and an approach to poetic power conveyed with thicker application of paint; and about 20 years later, from about 1865 onwards, his manner of painting became more lyrical, affected with a more impressionistic touch. In part, this evolution in expression can be seen as marking the transition from the plein-air paintings of his youth, shot through with warm natural light, to the studio-created landscapes of his late maturity, enveloped in uniform tones of silver. In his final 10 years he became the "Père (Father) Corot" of Parisian artistic circles, where he was regarded with personal affection, and acknowledged as one of the five or six greatest landscape Painters the world had seen, along with Hobbema, Claude Lorrain, Turner and Constable. In his long and productive life, he painted over 3,000 paintings.