A new photo exhibit from the storied Magnum Photo agency allows the public to view the contact sheets of 20 renowned photographers going back to the first days of the agency. That means viewers can see the shots that were taken before and after the photos that they've come to know so well.

The exhibition, which opened Thursday at the Milk Galleryin Manhattan, displays prints of the photos alongside the original session’s contact sheets—the pages of un-enlarged thumbnail prints that photo editors use to make selections. It's a rare look at the shooting process of some of the last century's best photographers.

“It’s really great to celebrate the old guard, the new guard, everything in between, to be able to look at the contact sheet and really see the labor that goes into capturing things and what that photographer was trying to convey and how they got there,” says Lauren Simon, who curated and organized the event as part of Magnum's annual general meeting.

Contact sheets have long been a staple of the photo process for publishers around the world. They’re often created as transfers made from direct physical and chemical contact with the negatives. As a result the original format dimensions are retained—a 35mm shot remains 35mm in size. They made for handy organizational and retouching tools, but they’re something of an artifact in the age of digital photos, functionally replaced by an SD card or Photoshop session.

Their borderline extinction is unfortunate since they can be quite revealing. For one, they show a more complete picture of a photographer's process than the final, chosen shot on its own. Guy Le Querrec's sheet from shooting Miles Davis in 1969 shows how many ideas he tried, flipping the frame and trying different compositions (it also shows that it was impossible to take an uncool picture of Miles Davis). Seeing the arc of a photo session with James Dean shows that it took a lot of posing to get those pictures with his trademark allure.

Even more, highlights and markings of the photographers and their editors hint at their photographic preferences in a way that the final image might not. The contact sheet that contains Elliott Erwitt's haunting photo of Jackie Onassis at her husband's funeral creates another level of the story. Among all the photos from that grim day, a single frame is sharply outlined in red, an ominous image in its own right.

"It’s very rarely a happy coincidence. It usually takes quite a bit of a trained eye to see what they’re getting at, but then being able to select and know which image has the balance, the composition, the right lighting," says Simon.

Magnum was founded by Henri Cartier-Bresson, a street photographer who famously championed a philosophy of what he called the decisive moment. He never cropped his images, and was known to suddenly snap pictures in a fluid and immediate way, rather than laboring over a subject until he got the “perfect shot.” He was also atypical—most photographers must work with their subjects for a period of time before managing to represent them the way they want.

“Sometimes the first shot you take isn’t always the best, sometimes it happens half way down the contact sheet, sometimes it’s the very last frame,” says Simon. “A lot of the iconic images came to be because, at the time, a photo editor at said magazine picked that image. Sometimes it’s a personal choice by someone years ago and now, when we get to reexamine these contact sheets, we get to see all these hidden treasures in them.”

The exhibition runs through July 13. Prints and several contact sheets are for sale, but even if you don't intend to buy anything it's worth a visit to see the photographic process laid out this way. Besides that, it's a group show of some of Magnum's top photographers, and that doesn't happen very often.

“I wanted to have a show up where we could have as much of the photographers’ work up as possible,” says Simon, “But being able to see these works all in one place—I think once it’s up on the wall I’m going to be a little starstruck, even though this is what I do all day.”

All images courtesy of Magnum Photos