Keep the original when the frame is part of the memory
The original ratio preserves the photographer’s first decision and is usually the safest starting point for landscapes, groups and scenes with important background detail. It also makes it easier to compare the color treatment without changing the composition at the same time.
Use it when the awkward edges or extra space contribute to the snapshot feeling. Not every retro image needs to be optimized for a platform first.
Use 1:1 or 4:5 for a tighter subject
A square crop gives a compact, centered feeling that suits portraits, objects and small moments. A 4:5 crop keeps more vertical room while still feeling deliberate, which can help a face or outfit occupy the frame without losing all context.
Crop from the center first, then move the subject only if the original composition clearly asks for it. The crop should support the gesture, not erase it.
Reserve 9:16 for vertical viewing
A 9:16 frame is useful for stories, phone wallpapers and full-screen vertical sharing. It asks more from the composition because empty side space disappears and the eye travels up and down instead of across.
Preview the effect after cropping. A timestamp, LCD frame or vignette may sit differently on a tall canvas, so make the final decision in the output ratio you actually plan to use.