"Tip for photos!"
This started as an added note to this post, but I found I had a lot to say on the subject. The original post was just about not taking photos of people without their permission, in any situation, but particularly if they look "funny" or "weird" to you. Strangely then added to it that this goes double for street performers, with which I wholeheartedly agree. People frequently feel comfortable taking photos or video of street performers, without affording those performers even the slim courtesy of a smile. This, I wish I needn't add, sucks.
One of the most frustrating aspects of being a street performer is being treated like an object. People take photos and video of you without tipping, without asking, without so much as smiling at you or staying to enjoy the show. They take your picture and walk away, and if feels like they've just stolen something.
I've even heard stories of professional camera crews setting up in front of a busker, blocking the view to others, and then leaving without so much as a dollar in the ol' guitar case. That's not just rude, that's defeating both purposes of the performance: to entertain others and make a little scratch for the performer.
If you absolutely MUST take a picture of a performer and feel like you can't ask without interrupting them, the absolute BARE MINIMUM courtesy you can extend them is to tip them. Doesn't matter if it's a dollar or the change you have in your pocket; the gesture counts as much. And if they have a sign with their name, take a picture of that too, so you can look them up later and offer them a copy of the photo if it turns out nice. The preferred method (by myself and most performers I know), is to ask permission (before or after), tip, and offer to provide a copy of the photo or video later.
Performing for free in the out of doors is fun, rewarding, and liberating. It allows people to experience art they never would have sought out on their own, or would not have access to for monetary reasons. It is an opportunity for performers to share their art, and for the public to show their appreciation with smiles, donations, or approbation. You're not required to watch, listen, or donate; you are required to treat us with the same courtesy you'd afford to anyone else.