Lots here on that and ethics thereof. Briefly, wide angle is obviously easier. But you can 'get permission' by making eye contact and moving your trigger finger up and down and 'ask' silently. Or like Winogrand, be ready, shoot fast & keep moving so they don't know or aren't sure it ever happened. Self-timer can be helpful, too. Or you can approach, tell them what you see, and enlist their help. All depends on the circumstances and how much of a people-person you are.