Why do movies or Netflix change aspect ratio?

Movies and Netflix content change aspect ratio for several reasons:

Artistic choices: Directors choose specific aspect ratios to match their creative vision. For example, widescreen formats (2.39:1) create a cinematic feel, while narrower ratios (1.85:1 or 16:9) might be used for intimate scenes.

Original filming format: Content is often filmed in different aspect ratios. Classic TV shows used 4:3, while modern productions typically use 16:9 or wider cinematic ratios (2.39:1).

IMAX sequences: Some films (like Christopher Nolan's movies) expand to taller aspect ratios for IMAX scenes, then return to widescreen for standard scenes, creating the black bar changes you notice.

Content protection: To fit widescreen content on standard screens, letterboxing (black bars top/bottom) or pillarboxing (black bars left/right) is added to preserve the original aspect ratio without cropping.

Platform optimization: Streaming services may adjust aspect ratios based on your device - phone screens get different framing than TVs to maximize the viewing area.

Historical content: Older movies and shows were filmed in 4:3 or Academy ratio (1.37:1), so they appear narrower on modern 16:9 screens.

Dynamic aspect ratio: Some directors intentionally change aspect ratios within a single film to create different emotional impacts or distinguish between different story elements (flashbacks, dream sequences, etc.).