The most common aspect ratio in still photography today is 3:2, which is based on the image size of 24x36mm captured on 35mm film cameras.
This image aspect has been carried over to digital photography with image sensors manufactured in the same aspect proportion, be it a full frame or APS-C sized sensors. Micro four-thirds and a range of compact digital cameras are fitted with 4:3 aspect ratio sensors. Current smartphones tend to capture images in the 16:9 aspect.
Quick Lesson 4: Understanding Image Aspect Ratio
3:2 Image Aspect Ratio
3:4 Image Aspect Ratio
1:1 Image Aspect Ratio
4:3 Image Aspect Ratio
5:4 Image Aspect Ratio
7:6 Image Aspect Ratio
16:9 Image Aspect Ratio
Many recent digital cameras come with a function that can change the aspect ratio of the image you are shooting. The availability of this function does permit a level of creative flexibility in the framing and composition of a photograph.
You must remember that if you are capturing the image in JPEG, the image will be cropped to that aspect and there is no reverse in the process. Shoot the image in RAW instead, where the image is saved as captured by the entire sensor and do your crop and other edits in post-processing.
Aspect Ratio: What it is and Why it Matters
An Introduction to Aspect Ratios and Compositional Theory