By Phil Coomes
BBC News website picture editor
One of photography's greatest strengths is the mystery of what is happening just out of the frame, and the moments in time either side of when the shutter is pressed.
Today thousands of digital frames languish on computers never to be seen but in the days of film, a contact sheet showing all the frames on a roll of film was the first stage from camera to publication.
These sheets can destroy the mystery and allure of the frozen moment, which is why many photographers keep theirs hidden. Who wants to open themselves up in that way, to show the discarded frames?
It has taken 50 years, but photographic students now have the chance to examine in detail every nuance of a book that holds an almost mythical place in the heart of many photographers.
The Americans by Robert Frank chronicles his 10,000-mile journey across America at the height of the Cold War, and today, many of the 83 frames contained inside the covers are burned into the collective memory.
The Americans was first published in France and then in the US. It was widely condemned by the critics, as it depicted a country sharply at odds with the America portrayed by Hollywood and the ad men.
Earl A Powell III, the director of the National Gallery of Art in Washington, says the book shows "a people plagued by racism, often ill-served by their politicians, intoxicated with the media and celebrities, and infatuated with speed, movement, and even the road itself".
Since publication there have been a number of revisions of The Americans and plenty of attempts to dissect and analyse the work. The latest is by Sarah Greenough whose book, Looking In: Robert Frank's "The Americans", accompanies a major exhibition at the National Gallery of Art in Washington.
Greenough draws on vintage contact sheets, work prints, and letters that chart Frank's journey around the US on a Guggenheim grant between 1955 and1956.
All of Frank's contact sheets that contain at least one of the pictures he selected for The Americans is reproduced. His raw outtakes lie alongside the fabled 83 photos.
These are not perfectly laid out sheets showing carefully presented negatives. Instead they are shown in jumbled rows, with frames upside down and some poorly exposed. And yet for all that they contain some of the most talked about frames in the history of photography.
To take just one example the contact sheet seen here contains two frames that made the final edit (13 and 16) as outlined in red.
Frank was shooting on a small 35mm camera without a motor, so unlike today where you'd find near identical frames sides by side, here we see how he has moved through the crowd, seizing moments of interest.
The shot of the bus (16) with the white and black faces split between the front and rear of the bus is one of two, the second being blurred and overexposed and lacking the race split.
The selected frame is both a social document and an example of the decisive moment, a split second when everything falls into place around the camera.
Other frames on the contact sheet that didn't make the cut include some that warrant further examination.
For example the black family in frame nine seems to have caught Frank's eye, and frame 15 is almost two pictures in one. The three rather dour looking women to the front would not be out of place in the book, as would the family pointing behind them.
Ultimately though, The Americans is a book that flows from front to back, a visual poem where sequencing was all important, and these frames just don't fit.
One that did - and a picture Frank has been known to describe as his favourite - is the crowd in frame 13. It could easily be overlooked as just a jumble of people, an easy shot.
Yet, next time you are out with a camera near a crowded street try taking a picture like this one. Every face is separated from the others. Almost no-one is looking at the camera. This is life passing in a frame. As Frank himself said of the picture, "the people walk this way and this way, they are young and they are old, they are fat and thin, everything Is there."
Perhaps that is the key and why Frank's work stands the test of time. These frames all look so rushed, so haphazard. And yet, they work so well, they have balance throughout each frame.
And this is the point - Frank had the desire and courage to throw away good frames. He had something to say, and that's the power of photography. To have a point of view at the heart of the work is what projects this photographic document to the top of the heap, even 50 years on.
Looking In: Robert Frank's The Americans is published by Steidl