Choosing a frame for a photograph
This page argues against roughly half the shelf for any given picture. That is the useful half of the advice.
Count the detail before you choose
The only reliable test is how much is already happening in the photograph. A market stall, a crowd, a forest, a plate of food: all of these are full of edges, and every one of those edges is competing for the same attention a decorative band wants. A single face against a plain wall, a building against sky, a flat-lay on white paper: these have space around them and can carry a band that does something.
Busy pictures take thin frames. A band of three or four per cent, in one flat colour, does the whole job — it tells the eye where the picture stops and then gets out of the way. Quiet pictures take wide ones, and can survive a pattern, a deep scallop or a glow without the result turning into noise.
The failure mode is easy to describe and hard to notice while you are making it: you widen the band because the frame looks better at that width, and stop looking at the photograph. If you cannot say what the picture is about after the frame goes on, the frame has won an argument it should have lost.