Some cameras have a fixed aspect ratio and others have ones that can be changed at will, so what should you choose?
I had better just explain what an aspect ratio is for anyone who's unsure. Simply, it's the shape, or format, of the image and more specifically the difference in size between the long and short edges of a picture. Your TV screen has a ratio of 16:9 so it's 16 units wide by 9 high. A square image has an aspect ratio of 1:1 as it's height and width are equal. We always use the lowest numbers to express the ratio, so we wouldn't say a TV was 32:18 (though technically it is). Old fashioned TVs (and computer monitors), many old movies, and some current cameras have a 4:3 ratio. Most SLRs and mirrorless systems are 3:2. Some cameras can be set to any of these.
original 3:2
So is it a bad thing if your camera has only one ratio? And if you have a choice which should you set and should you leave it at that? Both answers are a definite No. Unless you need to upload a specific size directly from your camera to the web, but that's rare.
The Crop tool is your friend. It gives you freedom but with it comes responsibility. Freedom of artistic choice but responsibility to make the most of your choice (and present the best for the viewer).
4:3
I'd like to take a step back, partly to put this into perspective and partly to show that, as photographers, we've never had it so easy. Hasselblad made some of the finest cameras in the film era (this is not a debate about who did or didn't, I'm using them as an illustration) that shot square images. Square is not a common format though some photographers make it work for them. The medium format square images were used in a whole host of commercial work and their sheer size allowed cropping to landscape or portrait as a publication required. Frame the original loosely and expand your options. The square format was very useful for album and CD artwork (remember them?).
16:9
Shooting transparencies on 35 mm film meant you had a 3:2 result. These could be masked down, even to a panoramic format, but you'd be losing a lot of what you'd captured. Shooting negatives, unless you could get prints on paper of the same aspect ratio, meant compromises. An 8x12 inch print would cover the whole frame whereas an 8x10 inch print meant loss of image at the sides. Th latter was a standard size for many years.
3:1 panoramioc
All of this assumes you want to use the complete frame and there are purists who stick to that. Of course, you can do your best to compose the scene to make the most of the format you were using. Thinking about your composition is no bad thing however. Some image though would look more pleasing or effective more letterbox or more square. Go for what you think is best and crop later. If you make a print you may need to trim the paper or make a bespoke aperture in some mount board. A4 and A3 prints are very close to the popular 3:2 format.
1:1 square
Conversely, a scene may offer alternative representations. Square, standard landscape, panoramic or even portrait can work equally well, of the same scene. Indeed in the ephotozine Critique Gallery all those options have been suggested using the same original upload. Neither are better or worse necessarily, just different though some will be better for some uses than others.
Portrait format alternative
The important point is not to be constricted. Web use especially doesn't restrict. Don't worry about displaying a square image on a widescreen TV if it shows off your image best. If the image is good people won't notice black borders.
Just a suggestion though, if you're going much narrower than 16:9 consider stitching multiple images to create your panorama.
All text and images © Keith Rowley 2020