Aspect Ratio
The proportional relationship between an image's width and its height.
Formula / rule
Ratio = width : height
Measured in
ratio
In depth
Every ratio has a temperament: 3:2 is the familiar 35 mm proportion, comfortable and slightly dynamic; 4:3 is calmer and better suited to portraits; 1:1 is static and formal; 16:9 is cinematic and emphasises horizontal movement. Choosing at the moment of capture is better than cropping later, because it changes how the scene is seen.
Examples in practice
Switching a camera to 1:1 for a portrait session changes the photographer's framing instinct before a single crop is made.