Which aspect ratio to use?
…the aim of this article is to focus on understanding the compositional impact of different aspect ratios, and more importantly, how to pick the right aspect ratio for a given subject.
There are two ways to go about this – either you shoot only one aspect ratio (for instance 3:2 because you have a digital SLR) and arrange the contents of your frame around it, or you keep an open mind and match the aspect ratio to respect the subject and the dynamic of your composition. For instance, you might use a 1:1 square for a round object if you want a balanced frame, or you might use 16:9 and just focus on one curve if you want to highlight a particular detail in a cinematic manner – you never see movie shots showing the whole of Earth, for instance; it’s always a hemisphere with the sun rising in just the right place. This is not a coincidence!
~Ming Thein from his article Aspect Ratios and Compositional Theory