The Impressionists, unlike their predecessors, were interested in painting the social life at the Opera, rather than painting solely what was on stage. The audience at the Opera included aristocrats (sometimes patrons of artists) and/or friends.  In a way this was the first attempt to create a "People's Magazine" in painting for those who attended the Opera.

This reinforces the fact that Impressionists were fascinated by the theater of social classes that surrounded them.

Mary Cassatt
The Loge 1882

This is one of the paintings that Cassatt presented for the fourth Impressionist  exhibition--her first showing with the Independents.

In the foreground, we see two socialites enraptured by the performance.  The curves of the balcony echo the curves on the fan and the curves of the women's shoulders.  The woman to the right holds a bouquet, as would the star of the performance.  She has a dreamy, introspective quality.  The woman right behind her strikes a theatrical pose. Her fan and her eyes are intent on the performance, but her body is leaning towards the woman in the foreground, in an act of public closeness.

Making the audience of an Opera performance the theme of a painting was a new  perspective  introduced by those who were rebelling against  the Salon.

Edgar Degas
Musicicans
c. 1869/72

This is a study of the Paris Orchestra by Degas is a study in light and movement,  influenced perhaps by Degas' interest in photography.

The expressionist strokes in between the players give dynamism to the composition.  It is interesting,  that, similar to photography, the person in the extreme foreground is blurred, while the musician in the middle ground is clear.

Focusing on musicians in the orchestra pit was also novel.  Degas experimented with theme and new styles of brushstrokes to create dynamism and movement, as well as a perspective seldom seen in Salon paintings.

Lautrec
Messalina at Bordeaux
c 1900

Lautrec, famous for his seedy cafe scenes tackles the Opera, no less...

What breaks the rules in this piece is the composition: the star of the performance is in the background, clearly provocative with her bright  red-orange dress.  In the forefront is a member of the ensemble, far larger and in greater detail than the star.  His muscular physiognomy is boldly portrayed, his head tilted towards the star of the show, but his gaze fixed on something else, away from the viewer.  His hand is perched on his sword, a symbol of eroticism.

The erotic tension between star and soldier is boldly portrayed by the diagonal composition joining soldier in the foreground with bright star in the background.   The red on the soldier's helmet, like the headdress of a cock, matches the red on the woman's dress.

Breaking the rules too, is point of view.  We are not looking at the performance  frontally,  but from the sidelines, as if we had gotten behind the curtains of the scene.