What’s depth of field? 

The area of an image where the subject is within the perfect point of focus. 

Shallow Focus? 

Shallow focus shots feature sharply defined foreground figures and blurry backgrounds. 

Deep Focus? 

Where all elements of an image (foreground, middleground, and background) are all in sharp focus. 

Shallow focus in Marie Antoinette 

In this case shallow focus is used to signal a pivotal moment for the character, whilst focusing the audience’s attention solely on them and their reactions since they are the only subject in focus.