This week The Wizard of Oz returns to theaters in a 3D IMAX presentation as part of the film's 75th anniversary. It's one of my favorite movies and one I've seen hundreds of times (twice this year in theaters, the optimal way to experience it), and every time I see it I find the film's transition from black and white* to Technicolor to be one of the most breathtaking moments in cinematic history. As a sepia-toned Dorothy opens the door of the sepia-toned Gale farnhouse the bursting, vibrant world of Oz explodes in lush and gorgeous color through the doorway. The camera tracks into Oz and Dorothy, now revealed to be wearing a bright blue gingham dress, steps over the threshold. It's a moment of true awe, and one of the most brilliant visual moments in the movies.

Today this scene would be easy to do digitally; colors are changed all over the frame in modern films. But there's no magic there. And in 1939 a bit of stage magic was needed.

The initial idea wasn't actually that far off from how it's done today, except it would have been accomplished by hand. Each frame would be sepia-toned by hand, until the door opened and the film transitioned into Technicolor. What can be done by a computer crunching numbers now would have taken hundreds of hours and tons of money back in 1939, so MGM abandoned the 'sepia stenciling' concept. Producer Mervyn LeRoy needed a new way to achieve the transition and he ended up using some good old fashioned sleight of hand.

He oversaw the film's extensive reshoots throughout the early months of 1939 (at one point the iconic song Somewhere Over The Rainbow was removed from the film because MGMs execs thought it was too slow, and so Dorothy's Kansas scenes needed to be reshot), and for the transition he used a simple trick. A set was painted sepia tone and Bobie Koshay, Judy Garland's double was outfitted in a sepia dress and given a sepia make-up job. Koshay walks to the door and opens it, revealing the bursting color of Munchkinland beyond the doorframe. She steps out of the way of the shot and the camera glides through the door, followed by Judy Garland, revealed in her bright blue dress.

It's a simple, elegant solution that makes the transition feel seamless. There's no sense of post-production trickery on the shot, just the wonderful expansion of the film's color palette. It's true movie magic.

* technically sepia. The Kansas sequences were shot in black & white and then dyed to a sepia tone to give an old-fashioned feeling to them.