Aspect ratios explained: 1:1, 4:3, 3:2, 16:9 and when to use them

What an aspect ratio is

An aspect ratio is the proportional relationship between width and height, written as two numbers: width first, then height. 1:1 is a square. 16:9 is wider than it is tall. 9:16 is the same shape turned on its side.

The ratio says nothing about size. A picture of 1920 × 1080 pixels and one of 3840 × 2160 pixels are both 16:9, because dividing width by height gives the same number, about 1.78. To find the ratio of any picture, divide both sides by the largest number that fits into each. 6000 × 4000 divided by 2000 gives 3:2.

Common ratios and where they come from

Each common shape has a history in a particular kind of camera or screen.

• 3:2: Canon describes it as the standard format of its EOS cameras and the shape of their image sensor. It is also the shape of the classic 4 × 6 inch print.
• 4:3: a slightly squarer shape. Canon notes that it is close to A4 paper, and cameras such as the Canon EOS R50 offer it as an option.
• 16:9: widescreen. YouTube's help pages give 16:9 as the standard aspect ratio for YouTube on a computer, with sizes such as 1920 × 1080 and 3840 × 2160.
• 9:16: 16:9 stood upright, the shape of a phone held vertically, used for vertical video.
• 1:1: the square. Canon links it to medium-format film cameras; today it is often chosen for profile pictures and product shots.
• 4:5: slightly taller than wide. It matches an 8 × 10 inch print, since 8:10 reduces to 4:5.

Changing the shape: crop, pad or stretch

When a picture has to fill a space of a different shape, there are three options, and they give very different results.

• Crop: cut away part of the picture so the rest has the new shape. Nothing is distorted, but whatever falls outside the box is gone. You choose what to keep.
• Pad (letterbox): keep the whole picture and fill the leftover space with bars of plain colour. Nothing is lost or distorted; the picture just appears smaller inside the frame.
• Stretch: force the picture to the new width and height. Every part is kept, but circles become ovals and faces look wider or thinner. It's almost never what you want.

Cameras can save a different shape than the sensor

Many cameras let you pick a shape before you take the picture. In the Canon EOS R50 manual, for example, JPEG photos are saved in the shape you select, such as 4:3, 16:9 or 1:1, while RAW files are always saved as 3:2, with the chosen shape recorded in the file. On playback, lines mark the chosen area inside the full 3:2 frame.

If you aren't sure how a picture will be used, shooting at the full shape and cropping later keeps your options open. You can always cut away; you can't add back what was never recorded.

Print sizes are ratios too

Photo print sizes are aspect ratios in disguise. When the photo's ratio and the paper's ratio differ, the print shop has to crop or add white borders, and the automatic crop may cut off someone's head.

• 4 × 6 in (10 × 15 cm): 2:3, the same shape as 3:2 turned upright.
• 5 × 7 in: 5:7, about 0.71, a little squarer than 2:3.
• 8 × 10 in: 4:5, noticeably squarer than a 3:2 photo.

Choosing a shape for a crop

Start from where the picture is going. Match the shape of the space and nothing is cut off or padded by someone else's software. For mixed uses, crop separate copies for each place rather than one compromise shape.

Leave room around the subject. A tight crop at one ratio can leave nothing to spare if you later need another. And remember that each landscape ratio has a portrait twin: 3:2 and 2:3, 4:3 and 3:4, 16:9 and 9:16.