Aspect ratio is the relationship between the width and the height of an image. It is written as two numbers, such as 3:2.
Most DSLR and mirrorless cameras with full-frame or APS-C sensors use 3:2, the same as 35mm film. Micro Four Thirds cameras, most compact cameras and most smartphones use 4:3. Other common ratios are 1:1 (square) and 16:9 (widescreen, as in video). Many cameras let you choose a different ratio, and you can also change it later by cropping.
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