Big heads are so called because only the head and shoulders of the subject are shown in the image. Shots of this type are generally full-face, or close to it, and perhaps the most searching disclosure of personality. They are created with the subject's knowledge at close range, they are intimate, and they invite engagement and communication. Eyes are usually the centres of interest, and expression and mood are key considerations. Lenses of focal length 85-135mm are ideal. The longer ones in this range compress perspective and give more space between camera and subject.
Some photographers dismiss head and shoulders shots as easy because there is no requirement to control the environment. This is valid to a point, but really good examples are difficult to capture. The main challenges are finding suitable light and novel compositions. Where possible, position the subject in diffused light and use a reflector or a touch of fill-flash to control contrast and put highlights in the eyes. Red-eye is not normally a problem provided the flash head is at least ten centimetres from the lens. Move around the subject to find the most flattering and interesting angles. It is not essential for the person's head to fill the frame, so explore ways of placing the subject off-centre and using space asymmetrically. Compositions of this type tend to be more interesting than standard centred shots.
Half profiles, where the subject turns slightly to one side, have more depth but depth of field may be limited in poor light. Focus on the eyes and control depth of field carefully. Remember that too much out-of-focus shoulder in the foreground may prove distracting.