CHRIS GAMPAT, THE PHOBLOGRAPHER:
"Modern autofocus is quite good–don’t get us wrong. But when push comes to shove and you need to capture a moment in a split-second, some focal lengths are easier to work with than others. Part of this has to do with depth of field at a specific distance and the other has to do with the specific focal length. For what it’s worth, a telephoto lens and a wide angle lens focused out to 7 feet away and shooting from the same distance will have a varying amount of the scene in focus. The telephoto will have less in focus while more of the scene will be in focus with the wider angle lens.
"But at the same time, you may not want to be so close to the scene that you’re within arm’s length of the person or scene being photographed. So to do that, we’re recommending three focal lengths that are best for candid photos."
Read more, here.