Is it okay to take a picture of a stranger in public?
My answer would be definitely not. It's never okay to take somebody's picture without telling them or without asking them for permission before you do it.
I think the respectable and the decent thing to do is to let that person know and make sure they're comfortable with you taking that picture. A lot of people hate having their picture taken by themselves or by others, so you don't want to put anybody in a difficult situation or make them feel uneasy about it.
Also, you should really let them know if you're going to be posting it up on any kind of social media, just let them know that other people are also going to see this picture and make sure they're familiar with what that means as well. That's the fair thing to do. So yeah, never take somebody's picture without permission.
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