On a crystal spring afternoon, atop a vertiginous spur on Mt Clark in Yosemite National Park, necessity and invention crossed the mind of Ansel Adams. He’d clambered to this dizzying spot to shoot pictures (just two, because they were all the plates he had left) of the sheer scarp of Half Dome. He shot one of the precious plates absolutely straight, exposing for an average of deep shadow and blinding snow. He knew it would print up exactly that—average.

“That was the first time I realised how the print was going to look,” he said later, “what I now call visualisation—and was actually thinking about the emotional effect of the image.” He loaded his last plate, and “began to visualise the black rock and deep sky. I really wanted to give it a monumental, dark quality.” So he switched the yellow filter for the only other he had brought with him, a No.29-F red filter that darkened the sky.

That day in 1927 marked the dawning of what Adams later called a “pointed awareness of the light”. That second plate of Half Dome printed up with deep blacks and sparkling whites—contrast that set off every rift and nuance in that vast granite monolith. His pictures were never the same again. They became famous for the exquisite realisation of a million tones, an approach to exposure we all nowadays know as the Zone System. To call it a trademark would be crass: it was a new way of seeing. Adams taught us to think outside the meter; to first stop and consider exactly how we wanted our images to look post-processing. What values we wanted in there, and how we wanted them to record. Just where they should fit on the tonal scale.

Light became Adams’ language, his dictionary, his palette.

For him, land and light became utterly inseparable and he understood intuitively not just that photography is, to quote David Doubilet, a great moment in light, but knew when it was writ beautifully before him.

He perceived those moments, and considered them a spiritual experience in a cathedral of wilderness.

This transcendental approach resonated with thousands, but Adams was not without his critics: documentary photographers and intellectuals often dismissed his work as a meek, irrelevant hobby. Henri Cartier-Bresson stung with, “The world is going to pieces and people like Adams and Weston are photographing rocks.” In fact, Adams’s work had monumental social impact—it is impossible to trace the birth and history of the environment movement without attributing him part-credit for bringing a wilderness ethic into the homes, galleries and minds of millions of Americans. Through his work, he advocated tirelessly for the backcountry, most prominently as one-time director and life member of the Sierra Club, one of America’s most powerful preservationist lobbies.

In The Tetons and the Snake River, shot in 1942, we can see Adams’ classic —and classical—approach to his work. He stood before that Wyoming scene and waited out the insipid grey of a rainy afternoon, trusting his instincts. When the light hit, he was ready, and he knew how to expose for it. Every ripple in the writhing Snake River is there. The beams in that spiritual sky flash Adams’ statement about wilderness—that the fates of people and places will inevitably cross.

Snake River is widely regarded as one of Adams’ greatest. In 2010, an original gelatin print auctioned by Christies fetched US $338,500.