If people know you take a picture of them they pose. Their faces and body language are different than when pictures are taken by surprise. The ultimate goal of portrait photography is to make people look natural, as if they are not aware of the photographer. From my own experience I know this is not always easy.
The idea of street photography is to take pictures of people in there normal habitat, just being themselves. Most of the time these pictures tell a better story than regular portrait photography where everything is arranged.
However, there is one issue about street photography that I like to address: privacy. Is taking pictures of people in a public space without asking for permission an intrusion in their privacy. In a sense it is. At the same time it is regarded as art. Look at the work of Henri Cartier-Bresson or Vivian Maier of very nice black and white pictures taken out on the streets. In my opinion it is essential not to embarrass people with pictures in awkward positions.
Street photography is not always easy with a full frame camera with a long lens. Everybody will see you pointing with your camera. Taking away the unexpected moment. Vivian Maier always used a small compact camera hanging around her neck. Therefore her pictures capture all the emotions in a very natural way. They are really storytelling pictures. And Henri Cartier-Bresson is of course famous for his “The Decisive Moment” with the famous picture of a man just about to step in a puddle of water.
Here are some of my street pictures taken in China.