Ask first
A smile and a gesture at the camera is enough, and it is almost always answered with a yes.
That question is the difference between a photograph of a person and a photograph of scenery. It usually opens a conversation, which is generally better than the picture. Offer to show them the frame afterwards — it costs nothing and it changes the encounter entirely.
One thing we would ask of anybody travelling with us: photograph people, not poverty.
Where it is restricted: inside certain temples and tombs, in military and border areas, and in some museums. Signs are not always in English and not always present, so when in doubt ask your guide rather than the guard. Drones are heavily restricted — assume the answer is no unless a permit was arranged well in advance.
