Photographing people in public requires a mixture of technical skill and the ability to beguile total strangers into posing. Nailing the technical side will help you make the most of your opportunities. When you make these decisions with confidence, on the fly, your chances of capturing those unforgettable faces are much improved.
1. Compose carefully
Imagine a set of four evenly spaced gridlines on your viewfinder. Two lines run horizontally, two vertically. By placing key objects in your photos at the intersections of these lines, you can guide viewers' eyes and keep them interested in your photos, even when the subject is not engaging directly with the camera.
2. Choose your focal length
Pre-set your lens to a distortion-free setting—e.g. 35mm—and then compose the picture you want to take with the time-honored method of physically moving forward until the frame is full. Trying to adjust composition by turning your lens's zoom ring is not the best way to compose a shot! Don't use a wide-angle lens—it will distort the subject.
3. Choose your depth of field carefully
Pre-set your aperture and shutter speed so you don't have to think about adjusting your camera dials while you're composing your best shot. As an example, shooting at ISO 100, an aperture of f/8 produces a shutter speed of 1/50th. f/8 ensures balance between two or more subjects; pushing the background slightly out of focus directs attention to your subjects. If the light's not changing, don't re-set your exposure between shots.