GETTING IT ON FILM #6 - WHY ARE 3D FILMS FLIPPED?

Sep 98 - Gord Harris


Have you ever wondered why we run 3D prints "flipped" - with the opposite emulsion position to regular 2D prints? The answer has to do with economics and practicalities of 3D film cameras and post production processes.

A normal 2D film camera runs with the negative film emulsion facing toward the lens, ready to capture the scene image. The exposed neg can be simply contact printed at the lab, emulsion to emulsion, such that the base side of the print faces the "lens side" of the image. When projected on a 1570 projector, the emulsion faces the projector lamp, and the base side of the film faces the field flattener and projection lens. This is the way it has always been in 2D, such that signs read properly and the sky is at the top as it should be!

Our early 3D camera rigs had the right eye camera facing straight ahead as for 2D photography, looking through a beamsplitter. Hence the right eye 3D print was projected as usual, base to lens and screen side. However, the left eye camera had to be positioned looking straight down into a 45 degree tilted beamsplitter, such that the image was reflected vertically and hence flipped top to bottom. To make this right side up again when projected, the left eye of the 3D print then had to be turned over, flipped top to bottom to match the right eye image. To make a long story short, this reversal of emulsion position in the left eye only caused numerous post production headaches and confusion about treating the left eye and right eye neg to print chain differently for editing, neg cutting, CGI effects and projection etc.

To improve this, the patented design configuration of the Imax 3D "Solido" camera treats both eyes the same - both images are flipped vertically. Two imaging mirrors deflect the images up and down to the film planes. The beauty of this arrangement was that contact prints can be made in the usual 2D manner, without expensive opticals, and both eyes treated the same through the entire editorial, post-production and projection chain. The images are simply made right side up again at the very end, in projection, by flipping both prints over. This is why the emulsion for 3D prints faces the screen, instead of the projector lamp. So now you know! Clear as mud? See the diagram.

(Reprinted from The IMAX Experience September/October 1998 Volume 2 Issue 5 with permission from IMAX Corporation)

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