Chris Foley Photo
A lot of Street Photographers have trouble with the notion of Street Portraits because asking somebody for a portrait, even via chance encounter, makes that a posed shot - which is not candid and therefore not "Street Photography." I say to hell with that. Street Portraiture is a perfectly valid subgenre of street photography (God I hate that very term..) and good things can come of asking permission. There are nuances, emotions, and responses that you simply cannot get from a candid. I'm not saying that a posed street portrait is any better than a candid shot, but that both have their place.
When I was sifting through images I made after stopping a stranger and asking them for their portrait I honestly thought I wasn't going to find very many of them, as I do mostly shoot candidly when working on the street. I was surprised to find over 200 examples of images I had taken with a stranger's permission and full participation. As you can see from some of these examples, it sometimes does pay to ask for a photo.
These situations were all different. Some of them yelled at me to come and take their picture. Others of them were very responsive and have since become friends, or at least social media friends. Some of them simply stopped for a moment and went off about their day. Others hammed it up for the camera in rather surprising ways. You never can tell what you're going to get, but the worst thing you can get is a "no."