Now, what is Documentary photography?
Essentially documentary photography is related to the role of press photographer, but it usually involves more in depth planning, researching and developing a story over a long period of time, rather than going out on quick assignments from wire services and newspaper editors as a regular press photographer would do. And whilst the press photographer use a single frame to tell his story a documentary photographer might use several pictures in a series to present his story.
Documentary photographers are nearly always freelancers, and their employers varies between newspapers, magazines or a book publisher. Or they might exhibit their work in a gallery. Their role is to tell a story trough pictures, whether these are photographs from a war zone, a study of old age, or images of a landscape changing because of development. But mainly photojournalists and documentary photographers explore and record human experience. Beginning with Henri Cartier-Bresson (regarded as the father of modern photojournalism), they have recorded significant moments in history, documented unfolding news and created images that have gained iconic status.
It is important to remember that we generally think of documentary photography as photography that has a connection to real situations and that it is supposed to represent those situations with minimal distortion. And even though you might think of photographs as honest evidence, we all know they can lie in many ways And this is where the integrity of the photographer comes in to play. The documentary photographer is expected to attempt to present work that minimizes distortion of reality and doesn't purposefully misrepresent.
Documentary photography is "all there is", in the sense that the goal is to make a record of observable reality at least as close as human technology will allow. Like a crime-scene photographer, one is ethically obliged to make an effort to record what is seen as accurately and completely as was possible. As a result of this documentary photographs usually have something candid to them.
Wikipedia describes candid photography as; un-posed and unplanned, immediate and unobtrusive. This is in contrast to classic photography, which includes aspects such as carefully staged portrait photography, landscape photography or object photography. And it could be argued that candid photography is the purest form of photojournalism. There is a fine line between candid photography and photojournalism, but this line have been blurred by photographers such as Henri Cartier-Bresson, Martin Parr and James Nachtwey amongst others.
So in conclusion, documentary photography is quite a broad sub-genre. It contains everything from architectural photography to candid shots that resembles street photography. Just keep in mind journalism's "five W's": who, what, when, where, why. Journalism is about telling a story. Most people experience the world at some twenty-four-odd frames a second, which is about 15 fps faster than most modern digital SLR's on the current market. And our brains, they process all of that information in real time. So a photograph, freezes the action, the moment, and forces our brains to experience reality as it never has before. It gives our brains a moment, to stop and smell the roses as it were.