The common 3:2 aspect ratio of most digital single lens reflex cameras was invented back in the 1920's by Oskar Barnack, when he designed the first 35 mm rangefinder for Leica. We have been stuck with this aspect ratio ever since. The reason I say stuck is because this aspect ratio can make composition a bit more difficult because it can easily have too much width in the landscape orientation and too much height in the portrait orientation.
The height is especially a problem for landscape photographers shooting vertical images because it often includes too much empty space in the ground or sky. For this reason, many film landscape photographers preferred the large format or medium format cameras because they used 5:4 and 4:3 aspect ratio respectively. These formats not only provided better quality for large prints but allowed for easier landscape compositions due to their aspect ratios.
Micro four thirds cameras offer in interesting alternative because they use the 4:3 aspect ratio. While other modern cameras try to solve this problem by allowing you to shoot the images in different aspect ratios. The only problem with this is they are accomplishing this trick by cropping, which means you are losing some resolution. With today's super high-resolution cameras like the 50 megapixel Canon 5DS you could easily accomplish the same thing by cropping the image in your image editing software and end up with a cropped image that still has high resolution.
The solution to the 3:2 aspect ratio for the DSLR shooter is going to depend on the situation. In general, do your best to fill the frame with your subject, but keep in mind you may need to crop to another aspect ratio in post-processing to eliminate the extra width or height. This may in turn affect your framing choice when taking the image. The other option is to fill the frame but the carefully compose your background. If the background of the image provides useful space in the image, it may not need to be cropped.
I have to make a correction to my video. The 1:1 aspect ratio doesn't work with the rule of thirds. The rule of thirds is based on the golden mean which is only useful for rectangles. Square images work well with centered subjects and subjects with shapes, such as circle, triangles, and rectangles. To learn more visit this article on how to shoot 1:1 aspect ratio images.
Click here to see more examples of 1:1 images.
Common Aspect Ratios:
Aspect ratio: image width/ image height, with the long dimension first.
There are six common aspect ratios for cameras today (and as many as you like if you use the crop tool, but that’s another subject for another day):
1:1 – Square format, traditionally the realm of 6x6cm Hasselblads, and now popularized by various mobile apps.
5:4 – Large format and sheet film cameras, mainly 8×10″.
4:3 – Broadcast television and video used this aspect ratio, originally in 640×480 pixel resolution; small sensor cameras and compacts (which inherited early video CCD architecture) have been using this aspect ratio ever since. Four Thirds and Micro Four thirds are the larger consumer formats to use it; in medium format there’s also 645 which has the same aspect ratio for both film and digital.
3:2 – Double a movie frame; famously invented when Oscar Barnack rotated the film through 90 degrees and doubled the width of the frame to create the 24x36mm ‘full frame’ 35mm camera format. Almost all larger sensored DSLRs use this today.
16:9 – HDTV format; not a native aspect ratio for digital still cameras, but useful to provide a more cinematic feel to an image.
2.35/2.40:1 – Motion picture widescreen for feature films; very rarely used for still photography, and there are certainly no dedicated digital still cameras that offer exclusively this format. Not only is it extremely wide, if you’re cropping down from a 4:3 sensor you’re throwing away more than half of your image.