When Luncheon on the Grass was first exhibited, it was met with significant controversy, mainly due to three main reasons criticized by the audience.

1. Nudity and Alleged Vulgarity: The painting depicted a nude woman lunching with two dressed men in a contemporary setting, considered highly scandalous at the time. The nudity of the female figure, juxtaposed with clothed men in a seemingly casual setting, challenged the traditional depictions of women in art, usually limited to heroic or mythological subjects. Critics argued that the painting was indecent, vulgar, and lacked decorum.
2. Challenging Classical Composition: Manet's composition deviated from the conventional rules of classical art. The characters were positioned in a non-hierarchical manner, and the painting lacked a clear narrative or a typical focal point. This unconventional arrangement disoriented the viewers accustomed to traditional academic compositions that guided their gaze and interpretation of the artworks.
3. Loose Brushwork and Flatness: Manet's brushwork can be described as loose and visible, unlike the smooth and blended technique preferred by academic painters. This painterly approach, combined with a lack of depth and modeling, resulted in a flatter appearance of the figures and the background. Critics argued that this technique lacked the polish and refinement expected in European art.

In breaking with the traditions of European art, Manet challenged the established norms and expectations of the academic art world. He rejected the idealized representations of women, historical and mythological subjects, and the refined technique of academic painting. Instead, Manet painted contemporary subjects, depicted them realistically, and used brushwork that revealed the process of painting. These departures from convention marked the emergence of Realism as an avant-garde movement and paved the way for numerous artistic innovations in the following decades.

Manet's Luncheon on the Grass broke with the traditions of European art by challenging not only the subjects depicted but also the techniques employed.

In terms of subject matter, the painting deviated from the prevalent themes in European art that focused on history, mythology, or religious narratives. Instead, Manet depicted ordinary people engaged in everyday activities, a stark departure from the idealized and romanticized portrayals of the time. The presence of a nude woman amidst clothed men in a contemporary setting caused a stir, as it challenged the established notions of decency and propriety in art.

Moreover, Manet's treatment of light and color also differed from the traditional approach. He employed a technique known as en plein air, where he painted outdoors to capture the natural light and atmosphere. This resulted in the use of broad brushstrokes, visible texture, and a more direct representation of the visual experience. The looseness of his brushwork and the lack of meticulous blending were considered unconventional and unrefined by the academic standards of the time.

Furthermore, Luncheon on the Grass exhibited a simplified and flattened pictorial space, with figures and objects lacking depth and three-dimensionality. This contrasted with the illusionistic depth achieved through perspective and modeling in traditional European art. The flatness of the composition further challenged the viewers' expectations of realistic representation and the hierarchy of classical painting.

Overall, Luncheon on the Grass not only unsettled viewers with its subject matter but also disrupted the established techniques and conventions of European art. Manet's bold approach and departure from tradition laid the foundation for the emergence of modern art, inspiring subsequent artists to explore new subjects, techniques, and approaches to representation.