Masking

Masking is a technique used to block out parts of an image. Masking can be achieved on the level of cinematography (i.e. masking the lens). Or, masking can be done through the mise en scène (i.e. a subject or object masking the background).

Masking is often used to suggest a point-of-view shot as seen through an optical device (i.e. binoculars, telescope, etc.).