How to decide what focal length to use in different situations ?

Focal length determines by how much ratio you want between environment and person.

Common mistake: choose focal length from person's size.

Correct way: choose focal length from how much environment you want, then move in a certain distance until you get the right person's size.

Situation to use focal length

24MM: Lots of environment, perfect for full body + extend leg + lower position. But don't put person on both size cause there will be distortion effect.

35MM: person + environment, perfect for Full body + 3/4 body.

50MM: person with some environment

70MM: portrait focus. chest up, face, and close up.