The aspect ratio of an image is the ratio of its width to its height. The larger the ratio, the ‘wider’ the image; the smaller the ratio, the ‘squarer’ the image.
Digital camera sensors have their own ‘native’ aspect ratio, and this is generally tied in to the sensor size. The small 1/2.3-inch sensors or similar in smartphones and point and shoot cameras have a 4:3 aspect ratio. This is also true of Micro Four Thirds cameras and medium format cameras.
The 1-inch sensors in some higher-end compact cameras have a wider 3:2 aspect ratio, as do APS-C and full frame camera sensors.
Very often, though, you need images in a different aspect ratio. Standard HD, full HD and 4K UHD video is captured in a much wider 16:9 ratio, the ratio now used for most TVs and computer monitors. This means the image captured by the sensor has to be cropped – you lose strips across the top and bottom edges. Some video cameras also capture slightly wider Cinema 4K (DCI 4K, C4K) video in the 19:10 ratio.
A few cameras have ‘multi aspect sensors’ (notably Panasonic) designed specifically to capture different aspect ratios with a more effective use of the sensor area so that there’s little or no loss in resolution when changing ratios.
This also affects the camera’s own LCD display (or viewfinder). Cameras designed for video will have a 16:9 display, but cameras designed for stills (and video) are more likely to have a 3:2 ratio display to match the sensor’s own aspect ratio – so in video mode, the display will have to be cropped with black bars top and bottom to show the wider 16:9 ratio.
It’s not just video where the required aspect ratio is not the same as the sensor’s. This happens all the time in stills photography too. If your images are being displayed online then the camera’s ‘native’ aspect ratio will be fine, as long as the website design allows for different aspect ratios. Very often, though, the way you’re displaying the photo will dictate the aspect ratio.
For example, if you want to post an image on Instagram, although the choice of ratios has now widened, most people still post square images, which have a 1:1 (square) aspect ratio. Or, if you want to display an image full-screen in a presentation or as a still in a video, it’s likely it will have to be in the 16:9 ratio.
Popular photo print sizes have different aspect ratios too. Postcard-sized 6” x 4” prints have the same aspect ratio as 3:2 sensors, which is handy, but 7” x 5” prints are slightly less wide and 10” x 8” prints are much ‘squarer’. A4 and A3 photo papers have the same ratio, which is half way between the 3:2 and 4:3 ratios used by most sensors.
The chief takeaway from all this is that different sensors have different aspect ratios and it’s likely that the images you capture will need a different aspect ratio again when you come to share, publish or print them. This means cropping what you’ve captured. With video, the camera does it, with stills, it’s something you do manually when you share/publish an image.