Portrait artists are like drill sergeants, commanding a few assistants to work on the lighting, the props, and the hair and makeup. When she was shooting big Vanity Fair group shots, Annie Leibovitz was more like a field commander, directing the efforts of dozens in composing the picture, making sure the subjects were posed in harmony with the others, and looking after myriad other small details.

The best party photographers, and their numbers are few, are like snipers. They fade into the background and grab their subjects in unposed assemblies, and most often without their knowing it.