Aspect ratio is the relationship between an image’s width and height, written as two numbers such as 16:9 or 4:3. It sets the shape of a video frame, how much of a scene fits without cropping, and how footage appears on a monitor, a phone or a video wall.
Aspect ratio is often confused with resolution. Resolution counts pixels; aspect ratio describes the rectangle those pixels fill, and the same rectangle can be produced at several resolutions.
Common ratios and what they suit
16:9 matches modern monitors and televisions, so it displays without black bars. 4:3 shows more height for a doorway or a corridor. Some cameras use a square or panoramic frame, and multi-sensor and fisheye devices can output shapes that need specific viewing software. The choice also decides how much of a wide scene a viewer takes in at once.
Cropping and playback
Changing the frame shape crops information. A 16:9 view squeezed into 4:3 loses the sides, and a panoramic image shown without dewarping looks distorted. Decide the ratio before the camera is mounted, because the fix afterwards usually costs effective resolution.