THE TECHNICAL DEFINITION
The aspect ratio is the relationship between a film frame's width and height, expressed as a number. The ratios you'll encounter most frequently: 1.33:1 (the old 4:3 television and early cinema format), 1.78:1 (the 16:9 widescreen television standard), 1.85:1 (the American theatrical widescreen standard, used by most Hollywood studio films), and 2.39:1 (CinemaScope anamorphic widescreen, the widest common theatrical format). The number before the colon is the width; the number after is always 1, representing the height.
The technical description is accurate and almost useless for understanding what matters. What matters is what the shape of the frame does to your experience of watching - how the proportions change your relationship to the subjects within them, what each ratio emphasises, and why filmmakers choose the shapes they choose.
WHAT DIFFERENT RATIOS DO
The wider the frame, the more it emphasises horizontal relationships: the relationship between figures and landscape, the visual tension between characters occupying opposite sides of the frame, the environmental scale that makes individuals look small within it. 2.39:1 is a ratio built for epic landscapes, the vast scale of the American West, the formal rhetoric of grandeur. Leone's Spaghetti Westerns. Lawrence of Arabia. Any film that wants the environment to dwarf the person in it.
The squarer the frame, the more it emphasises the individual, the face, the vertical relationship between figures. 1.33:1 - the old television ratio, the early cinema ratio - is an intimate format, built for faces rather than landscapes. Certain filmmakers have returned to it deliberately as a formal choice: Gus Van Sant's Elephant, Andrea Arnold's American Honey, Pawel Pawlikowski's Ida all use the squarer format to create a specific intimacy, a sense of the frame pressing close to its subjects.
FILMMAKERS WHO USE RATIO AS ARGUMENT
Wes Anderson uses aspect ratio changes explicitly as formal argument in The Grand Budapest Hotel (2014). Three different time periods in the film use three different aspect ratios - 2.35:1 for the contemporary framing narrative, 1.85:1 for the 1980s hotel, and the square 1.33:1 for the 1930s hotel heist at the film's core. The ratios encode the historical periods: the older the memory, the squarer and more intimate the frame. The film is formally about the past as a construct; the ratio choices make that argument visible.
Xavier Dolan changes aspect ratio mid-film in Mommy (2014) as an emotional argument. The film begins in a constricted 1:1 square format - the frame pressing on its subjects, the limited visual space enacting the claustrophobic circumstances. In one scene, the protagonist opens his arms and the frame expands to 2.39:1 in real time, the world opening. Then it contracts again. The aspect ratio as an emotional state.
Paul Schrader shoots First Reformed (2017) in 1.33:1 - the squarest of the common ratios - for a film about a crisis of faith, isolation, and the specific loneliness of a man who has sealed himself into a rigid formal system. The frame presses. The world outside the frame is implied but not shown. The ratio is the theology.
THE BLACK BARS
The black bars that appear above and below a widescreen film on a standard television screen are not a problem - they are the correct presentation of the aspect ratio. The alternative - zooming into the image to fill the screen - removes information from the sides of the frame. Many widescreen films are composed with that information being formally meaningful: faces positioned at the extreme edges, spatial relationships established across the width of the frame.
'Widescreen' means wider than your screen. The bars are the proof that you're watching the film in the ratio the filmmaker chose. When you see them, they're correct. When you don't see them on a widescreen film, information has been cropped.
The bars are the proof that you're watching the film in the ratio the filmmaker chose.